Engine Controls Self-Diagnostics - 6.0L - Bi-Fuel & Cng: Diagnosis Chevrolet Silverado 2500

ON-BOARD DIAGNOSTICS

The Alternative Fuels Engine Control Module (AF ECM) is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, the AF ECM will store a Diagnostic Trouble Code (DTC) and, if code is emissions related, illuminate the Malfunction Indicator Light (MIL) located on instrument cluster.

There are 2 paths for accessing on-board diagnostics. It is necessary to access information through both paths, as each path presents some different information.

1. Generic OBD-II This provides all generic codes and some OEM codes, serial data PIDs required for generic OBD-II and some OEM PIDs, monitor status, pending codes and freeze frame.
2. OEM All DTCs, all PIDs, failure records, enhanced scan tool information and all other OEM scanner functions.

Scan Tool Snapshot

The scan tool can be set up to take a snapshot of the parameters available via serial data. The snapshot function records live data over a period of time. The recorded data can be played back and analyzed. The scan tool can also graph parameters singly or in combinations of parameters for comparison. The snapshot can be triggered manually at the time the symptom is noticed, or set up in advance to trigger when a DTC sets. An abnormal value captured in the recorded data may point to a system or component that needs to be investigated further. Refer to scan tool user instructions for more information on the Snapshot function.

DIAGNOSTIC PROCEDURE

Diagnosis of computerized engine control system should be performed in the following order

1. Ensure all engine systems not related to computer system are operating properly. DO NOT proceed with testing unless all other problems have been repaired. Diagnostic system check must be performed before using specific DTC testing procedure. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) .
2. If no DTCs are present and a driveability problem exists, see SYMPTOMS in appropriate TROUBLE SHOOTING - NO CODES article. Doing so will help identify proper system or component to check in appropriate SYSTEM & COMPONENT TESTING article.
3. After necessary repairs are made, clear DTCs, verify vehicle will enter "closed loop" operation and ensure DTC does not reset.

RETRIEVING DIAGNOSTIC TROUBLE CODES

DTCs are retrieved using a Tech 2 scan tool, or other OBD-II compatible scan tool connected to OBD-II 16-pin Data Link Connector (DLC). (Scheme 58) See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS. DLC is located below left side of dash.

DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS

Note. Check for applicable Technical Service Bulletins (TSB) before proceeding. DO NOT perform the following procedure if driveability conditions are not present. DO NOT turn ignition off during the following procedure or clear DTCs unless instructed to do so.

Note. DO NOT perform this test if no driveability condition exists. Search for applicable bulletins before proceeding with the diagnosis. DO NOT turn OFF the ignition when performing this diagnostic test. DO NOT clear the DTCs unless the diagnostic instructs you to do so. If there is a condition with the starter system, see appropriate STARTERS article in STARTING & CHARGING SYSTEMS. Ensure the battery is fully charged and the cables are clean and tight. Ensure the PCM and the AF ECM grounds are clean, tight, and in the proper location. Ensure the CNG and the gasoline fuel tanks are at least 1/2 full.

1. Install a scan tool. Does the scan tool turn ON? If yes, go to next step. If no, see SCAN TOOL DOES NOT POWER UP in appropriate BODY CONTROL MODULES article in ACCESSORIES & EQUIPMENT.
2. Attempt to establish communication with the AF ECM with a scan tool. Does the scan tool communicate with the module? If yes, go to next step. If no, see «DTC P0600: CLASS 2 SERIAL DATA CIRCUIT»(ref-151060-S28866912942002120900000) .
3. Attempt to establish communication with the listed control modules. If you are using a Tech 2, obtain the information with the Class 2 Message Monitor feature: The Instrument Panel Cluster (IPC). The Powertrain Control Module (PCM). The Body Control Module (BCM). Does the scan tool communicate with all the listed control modules? If yes, go to next step. If no, see SCAN TOOL DOES NOT COMMUNICATE WITH CLASS 2 DEVICE in appropriate BODY CONTROL MODULES article in ACCESSORIES & EQUIPMENT.
4. Attempt to start the engine. Does the engine start and idle? If yes, go to next step. If no, see «NO-START - ENGINE CRANKS OKAY (4.8L, 5.3L & 6.0L)»(ref-149936-S25063397352002110100000) under NO-START DIAGNOSIS in BASIC DIAGNOSTIC PROCEDURES - TRUCKS - EXCEPT TRACKER article.
5. Select the DTC display function for the following control modules: The AF ECM. The BCM. The EBCM. The IPC. The PCM. The HVAC. Does the scan tool display any DTCs? If yes, go to next step. If no, go to step If yes, go to step 11 .
6. With a scan tool, select Captured Info in order to store the powertrain DTC information. When complete, go to next step.
7. Does the scan tool display DTCs which begin with a "U"? If yes, see SCAN TOOL DOES NOT COMMUNICATE WITH CLASS 2 DEVICE in appropriate BODY CONTROL MODULES article in ACCESSORIES & EQUIPMENT. If no, go to next step.
8. Were any of the PCM DTCs P0601, P0602, or P1621 set? If yes, see «DTC P0601-P0607, P1600, P1621, P1627, P1680, P1681 & P1683: PCM MEMORY»(ref-138163-S05643150422002032500000) under DIAGNOSTIC TESTS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C" & "K" SERIES TRUCKS. If no, go to next step.
9. Does the scan tool display DTC P0562, P0563, P1637, or P1638? If yes, see appropriate GENERATORS & REGULATORS article in STARTING & CHARGING SYSTEMS. If no, go to next step.
10. Compare the stored PCM DTCs to the stored AF ECM DTCs. Diagnose the DTCs in the following order: If the same DTC is stored in the PCM and the AF ECM, refer to the alternative fuels DTC information. If a DTC is stored only in the PCM, refer to the applicable gasoline DTC information. If different DTCs are stored in the PCM and the AF ECM, diagnose the PCM DTCs first. Does the scan tool display any PCM or alternative fuels DTCs? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, go to next step.
11. Observe the Malfunction Indicator Lamp (MIL). Is the MIL continuously illuminated? If yes, go to next step. If no, see «MIL INOPERATIVE»(ref-151060-S00741190122002120600000) under SYSTEM TESTS.
12. Command the MIL OFF with the scan tool. Observe the MIL. Is the MIL OFF? If yes, go to next step. If no, see «MIL ALWAYS ON»(ref-151060-S13326205702002120600000) under SYSTEM TESTS.
13. Is the vehicle equipped with the dedicated CNG option RPO KL8? If yes, go to step 17 . If no, go to next step.
14. Observe the AFO Signal and AFO Signal Command data display with the scan tool. Do both parameters display AFECM? If yes, go to next step. If no, see «FUEL SYSTEM UNWANTED SWITCH TO GASOLINE (BI-FUEL ONLY)»(ref-150429-S37033910182002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
15. Observe the Fuel Indicator Lamp (FIL). Is the FIL illuminated? If yes, see FUEL INDICATOR LAMP ALWAYS ON - ALTERNATIVE FUELS in appropriate ANALOG INSTRUMENT PANELS article. If no, go to next step.
16. Start and idle the engine. Force a switch over to gasoline operation with the scan tool in the AFECM special functions, output controls menu. Does the engine continue to idle? If yes, go to next step. If no, see «ALTERNATIVE FUEL PUMP RELAY»(ref-150429-S28839994002002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
17. Is the customer's concern with the transmission? If yes, see «DIAGNOSTIC SYSTEM CHECK»(ref-133047-S18769834712002021100000) under SELF-DIAGNOSTIC SYSTEM in HYDRA-MATIC 4L80-E DIAGNOSIS - AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article. If no, go to next step.
18. Is the customer's concern with the Inspection/Maintenance (I/M) testing? If yes, see «INSPECTION/MAINTENANCE SYSTEM CHECK»(ref-138163-S31903009882002032500000) under INSPECTION/MAINTENANCE PROCEDURES in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C" & "K" SERIES TRUCKS article. If no, go to next step.
19. If the customer's concern is a symptom, see «TROUBLE SHOOTING - NO CODES - TRUCKS»(ref-149956) article, or Fuel Gauge Inaccurate or Inoperative - Alternative Fuels or Fuel Indicator Lamp Inoperative - Alternative Fuels in appropriate ANALOG INSTRUMENT PANELS article. Did you find and correct the condition? If yes, system is okay. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) .

CLEARING DIAGNOSTIC TROUBLE CODES

There are 3 methods to clear DTCs.

Scan Tool

Scan tool is preferable way to clear DTC information. Freeze frame and failure record data will also be cleared. PCM adaptive learning and system monitors will only be cleared for those system which had a stored DTC.

SCAN TOOL USAGE

Scan tool is a specialized tester which, when connected to DLC, can be used to diagnose on-board computer control systems by providing instant access to circuit voltage information without need to crawl under dash or hood to backprobe sensors and connectors.

Scan tool reduces diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. They may also furnish information on output device (solenoids and motors) status. However, status parameters only indicate output signals have been sent to devices by PCM; they do not indicate whether devices have responded properly to signal. Verify proper response at output device using a voltmeter or test light.

A problem may exist even if DTCs are not present. Many driveability problems occur without setting DTCs.

Using a scan tool is the easiest method of checking sensor specifications and other data parameters. Scan tool is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters.

Note. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If nonexistent codes are displayed, DO NOT use scan tool for diagnosis. Contact tester manufacturer for additional information.

Scan Tool Display

Note. OBD-II vehicles have options available in the scan tool DTC mode to display enhanced information available. However, to fully utilize information and procedures requires the use of a Tech 2 scan tool. See scan tool operator's manual for additional information.

The following are Tech 2 scan tool sub-menus in the DTC INFO and SPECIFIC DTC modes

1. DTC INFO MODE Used to search for specific type of stored DTC information. There are 7 choices in this mode. Technician may be instructed to test DTCs in a certain manner. Follow the affected DTC test procedures. To get complete description of any status, hit ENTER key before pressing the desired "F" key.
2. DTC STATUS This selection will display any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition up to a maximum of 33 DTCs. A DTC test that runs and passes will cause affected DTC to be removed from scan tool screen.
3. FAIL THIS IGN. This selection will display all DTCs that have failed during the present ignition cycle.
4. HISTORY This selection will display only DTCs that are stored in the control module's history memory. It will not display type "D" DTCs. It will display all type "A" and type "B" DTCs that have the MIL illuminated and have failed within the last 40 warm-up cycles. It will also display type "C" DTCs that have failed within the 40 warm-up cycles.
5. LAST TEST FAIL This selection will only display DTCs that have failed the last time the test ran. If type "A" or "B" DTCs are displayed, the last test may have ran during the previous ignition cycle. For type "C" DTCs, the last failure must have occurred during the current ignition cycle to be displayed as LAST TEST FAIL.
6. MIL REQUEST This selection will only display DTCs that are requesting MIL illumination. Type "C" DTCs cannot be displayed using this option. This selection will report type "B" DTCs only after MIL illumination has been requested.
7. NOT RUN SCC Not Run Since Code Clear option will display up to 33 DTCs that have not run since DTCs were last cleared. Since any displayed DTCs have not run, their condition (passing or failing) is unknown.
8. TEST FAIL SCC Test Fail Since Code Clear selection will display all active and history DTCs that have reported a test failure since the last time DTCs were cleared. DTCs that last failed more than 40 warm-up cycles before this option is selected will not be displayed.
9. FAILED SINCE CLEAR This message indicates the DTC has failed at least once within the last 40 warm-up cycles since the last time DTCs were cleared.
10. NOT RUN SINCE CL. Not Run Since Cleared message indicates that the selected diagnostic test has not run since the last time DTCs were cleared. Therefore, the diagnostic test status (passed or failed) is unknown. After DTCs are cleared, this message will continue to be displayed until the diagnostic test runs.
11. NOT RUN THIS IGN. Not Run This Ignition message indicates the selected diagnostic test has not run this ignition cycle.
12. TEST RAN AND PASSED This message indicates the selected diagnostic test has: Passed the last test. Ran and passed during this ignition cycle. Ran and passed since DTCs were last cleared. Test has not failed since DTCs were last cleared. If this message is displayed, repair is complete. If FAILED THIS IGN. message is displayed, repair is incomplete and further diagnosis is required.

INSPECTION/MAINTENANCE SYSTEM CHECK

For inspection/maintenance system check procedures, see INSPECTION/MAINTENANCE SYSTEM CHECK under INSPECTION/MAINTENANCE PROCEDURES in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C" & "K" SERIES TRUCKS article.

DIAGNOSTIC TROUBLE CODE DEFINITIONS

Note. Use of Tech 2 or generic OBD-II scan tool is required to retrieve DTCs. Refer to user reference manual supplied with scan tool.

DIAGNOSTIC TROUBLE CODE DEFINITIONS

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Diagnostic Procedures

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. Is the MIL continuously illuminated? If yes, go to next step. If no, go to step 4 .
3. With the scan tool in the Alternative Fuels - Special Function menu, command the MIL OFF. Is the MIL illuminated? If yes, see «MIL ALWAYS ON»(ref-151060-S13326205702002120600000) . If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Crank the engine for at least 10 seconds. Does the engine start and run? If yes, go to next step. If no, see NO-START - ENGINE CRANKS OKAY in BASIC DIAGNOSTIC PROCEDURES - TRUCKS - EXCEPT TRACKER article.
5. Observe the operation of the other Instrument Panel (IP) gauges and/or lamps. Do all the IP indicators, except the MIL, operate correctly? If yes, go to next step. If no, see appropriate ANALOG INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT.
6. Turn the ignition ON with the engine OFF. Observe the scan tool MIL Input - Gasoline/PCM data parameter in the Alt. Fuels Data List. Is a gasoline MIL Input request present? If yes, go to next step. If no, go to step 9 .
7. Turn the ignition OFF and wait 30 seconds. Disconnect the AF ECM connectors. Turn ON the ignition leaving the engine OFF. Connect a 3-amp fused jumper between the AF ECM MIL control circuit terminal and a known-good ground. If the jumper fuse opens repair the short to voltage on the AF ECM MIL control circuit. Is the MIL illuminated? If yes, go to step 11 . If no, go to next step.
8. Test the MIL control circuit between the AF ECM connector terminal and the IP for an open. Did you find and correct the condition? If yes, go to step 14 . If no, see appropriate ANALOG INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT.
9. Test the MIL input circuit between the AF ECM connector terminal and the gasoline PCM connector terminal for an open or a short to voltage. Did you find and correct the condition? If yes, go to step 14 . If no, go to next step.
10. Inspect for proper terminal contact at the PCM connector. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 12 .
11. Inspect for proper terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 13 .
12. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 14 .
13. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
14. Remove all test equipment except the scan tool. Connect any disconnected components, fuses, etc. With a scan tool, clear DTC information from the AF ECM and the PCM. Turn the ignition OFF and wait 30 seconds. Start the engine and operate the vehicle. Observe the MIL. Observe vehicle performance and driveability. After operating the vehicle inspect BOTH control modules for stored DTCs. Does the vehicle operate on CNG with normal driveability, no MIL illumination and no stored DTCs? If yes, system is okay. If no, «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. Is the MIL continuously illuminated? If yes, go to next step. If no, see «MIL INOPERATIVE»(ref-151060-S00741190122002120600000) .
3. With the scan tool in the Alternative Fuels - Special Function menu, command the MIL OFF. Is the MIL illuminated? If yes, go to step 5 . If no, go to next step.
4. Observe the DTC information from the PCM. Observe the DTC information from the AF ECM with a scan tool. Does the scan tool display any MIL Request DTCs? If yes, see «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
5. Observe the MIL input (Gasoline/PCM) data parameter in the Alt. Fuels Data List while the MIL is commanded OFF. Is a gasoline MIL input request present? If yes, go to step 7 . If no, go to next step.
6. Test the MIL control circuit between the AF ECM connector terminal and the IP for a short to ground. Test the IP for an internally shorted condition. Repair the circuit or IP as necessary. Did you find and correct the condition? If yes, go to step 11 . If no, go to step 9 .
7. Test the MIL input circuit between the AF ECM connector terminal and the gasoline PCM connector terminal for a short to ground. Repair the circuit as necessary. Did you find and correct the condition? If yes, go to step 11 . If no, go to next step.
8. Connect the AF ECM connectors. Disconnect the PCM connectors. Turn ON the ignition with the engine OFF. DTCs may set with the PCM disconnected. Clear DTCs after all diagnosis is complete. Observe the MIL input (Gasoline/PCM) data parameter in the Alt. Fuels Data List. Is a gasoline MIL input request present? If yes, go to next step. If no, go to step 10 .
9. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to step 11 .
10. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
11. Remove all test equipment except the scan tool. Connect any disconnected components, fuses, etc. With a scan tool, clear DTC information from the AF ECM and the PCM. Turn the ignition OFF and wait 30 seconds. Start the engine and operate the vehicle. Observe the MIL. Observe vehicle performance and driveability. After operating the vehicle inspect BOTH control modules for stored DTCs. Does the vehicle operate on CNG with normal driveability, no MIL illumination and no stored DTCs? If yes, system is okay. If no, «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM.

DIAGNOSTIC TESTS

Note. Before clearing DTCs, perform powertrain diagnostic system check. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM. Record FREEZE FRAME and FAILURE RECORDS for reference during testing. Data will be erased when DTCs are cleared.

Conditions For Running DTC

1. The engine is operating on alternative fuel.
2. No AF ECM, ECT, TP, IAT, MAP, or MAF sensor DTCs are set.
3. The engine has been operating for more than 10 seconds.
4. The engine speed is more than 500 RPM.
5. The battery voltage is more than 10 volts.
6. The MAP sensor performance DTC P0106 has run at least once this ignition cycle.

Conditions For Setting DTC

The AF ECM monitors an actual MAF sensor air flow rate that does not match a calculated air flow rate for at least 5 seconds continuously.

Action Taken When DTC Sets

The AF ECM will switch-over to gasoline fuel operation. Alternative fuel operation will not be attempted until the next ignition key cycle.

1. The AF ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
2. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive warm-up cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Start the engine. Observe the DTC information from the AF ECM and the PCM with a scan tool. Does the scan tool display any other DTCs? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «DIAGNOSTIC AIDS»(ref-151060-S13289755352002120600000) .
4. Idle the engine. Observe the MAP sensor voltage parameter with a scan tool. Does the scan tool indicate that the MAP sensor voltage is less than 0.8 V? If yes, see «DTC P0107: MAP SENSOR CIRCUIT - LOW VOLTAGE»(ref-151060-S25300056622002120600000) . If no, go to next step.
5. Does the scan tool indicate that the MAP sensor voltage is more than 4.0 V? If yes, go to step «DTC P0108: MAP SENSOR CIRCUIT - HIGH VOLTAGE»(ref-151060-S36790358322002120600000) . If no, go to next step.
6. Turn ON the ignition, with the engine OFF. Observe the MAP sensor kPa parameter with a scan tool. Compare the MAP sensor kPa parameter to a known-good vehicle. Is the MAP sensor kPa parameter within 3.0 kPa of the known-good vehicle? If yes, go to next step. If no, see «DTC P0106: MAP/TP SENSOR PERFORMANCE/RANGE»(ref-151060-S04390065212002120600000) .
7. Observe the MAP sensor voltage parameter with a scan tool. Start the engine. Does the MAP sensor voltage parameter change value after you started the engine? If yes, go to next step. If no, see «DTC P0106: MAP/TP SENSOR PERFORMANCE/RANGE»(ref-151060-S04390065212002120600000) .
8. Turn ON the ignition, with the engine OFF. Observe the TP sensor voltage parameter with a scan tool. Press the accelerator pedal from a closed throttle position, to a wide open throttle position. Does the TP sensor voltage parameter go from below 1.0 V to above 4.0 V? If yes, go to next step. If no, see «DTC P0121: TP SENSOR ANGLE OUT OF RANGE»(ref-151060-S03695337972002120600000) .
9. Inspect for the following conditions: MAF sensor wiring harness improperly routed. An intake air duct leak. A collapsed or damaged intake air duct. An intake manifold or throttle body vacuum leak. Debris near the MAF sensor inlet screen or damage to the inlet screen. An improperly installed MAF sensor. Inspect the flow arrow for proper sensor installation. A Positive Crankcase Ventilation (PCV) system leak. An incorrect PCV valve, or faulty operation. A loose or missing oil fill cap. Repair any condition as necessary. Did you find and correct the condition? If yes, go to step 14 . If no, go to next step.
10. Turn OFF the ignition. Disconnect the harness connector of the MAF sensor. Turn ON the ignition, with the engine OFF. Connect a test lamp between the ignition voltage circuit of the MAF sensor and a good ground. Connect a DMM to the probe of the test lamp and a good ground. Does the voltage measure near battery voltage? If yes, go to next step. If no, go to step 12 .
11. Test for an intermittent and for a poor connection at the MAF sensor. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 13 .
12. Repair the high resistance in the ignition voltage circuit of the MAF sensor. After repairs, go to step 14 .
13. Replace the MAF/IAT sensor. See «MASS AIRFLOW SENSOR»(ref-150199-S12133959972002110600000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
14. Use the scan tool in order to clear the DTCs. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
15. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

Diagnostic Aids

Inspect for the following conditions

1. A mis-routed harness. Inspect the MAF sensor harness for routing that is too close to the following components: Secondary ignition wires or coils. Solenoids. Relays. Motors.
2. The throttle plate for sticking, or for deposits on the throttle plate, or in the throttle bore.
3. Any un-metered air that enters the engine may cause this DTC to set. Make sure that the oil dip stick is seated, and the oil fill-cap is secure.

For an intermittent condition, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.

Cranking Test

1. The engine is cranking on alternative fuel, but has not yet started.
2. The battery voltage is more than 8 volts.

Running Test

1. The engine is operating on alternative fuel.
2. The engine has been operating for more than 10 seconds.
3. The battery voltage is more than 10 volts.
4. The engine speed is more than 500 RPM.

Cranking Test

1. The AF ECM monitors a MAF sensor signal of less than 125 Hz (0 gms/sec) for at least 0.5 second continuously.

Running Test

1. The AF ECM monitors a MAF sensor signal of less than 1500 Hz (2.5 gms/sec) for at least 0.5 second continuously.

Actions Taken When DTC Sets

The AF ECM will switch-over to gasoline fuel operation. Alternative fuel operation will not be attempted until the next ignition key cycle.

1. The AF ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
2. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions For Clearing MIL/DTC

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
3. Start and idle the engine. With a scan tool, establish communication with the Powertrain Control Module (PCM). Observe the MAF sensor data parameter. Does the MAF sensor data parameter indicate airflow less than 2.5 gms/sec? If yes, go to next step. If no, go to step 7 .
4. Turn OFF the ignition. Disconnect the MAF/IAT sensor connector. Turn ON the ignition, with the engine OFF. Measure the voltage from the signal circuit of the MAF sensor to a good ground with a DMM. Is the voltage near 5 V? If yes, go to step 8 . If no, go to next step.
5. Inspect the MAF sensor signal circuit for an open or high resistance between the sensor connector and the PCM. Inspect the MAF sensor signal circuit for a short to voltage if the circuit voltage was more than 5.2 volts. Inspect for proper terminal contact at both the PCM connector and the MAF sensor connector. Did you find and correct the condition? If yes, go to step 16 . If no, go to next step.
6. Inspect the MAF sensor signal circuit for a short to engine ground or to the sensor ground circuit. Disconnect the AF ECM connector containing the MAF sensor signal circuit in order to determine if the AF ECM is the cause of the short. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 14 .
7. Inspect the MAF sensor signal circuit for an open or high resistance between the circuit splice and the AF ECM connector. Inspect for proper terminal contact at the AF ECM connector. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 15 .
8. Connect a test lamp between the ignition feed circuit terminal and the ground circuit terminal of the MAF sensor connector. Does the test lamp illuminate? If yes, go to step 12 . If no, go to next step.
9. Connect the test lamp between the ignition feed circuit terminal of the MAF sensor connector and a known-good ground. Does the test lamp illuminate? If yes, go to step 11 . If no, go to next step.
10. Repair the cause of no MAF sensor voltage. Possible causes include the following: An open ignition fuse. An open or shorted to ground ignition feed circuit. A shorted to ground component that shares the MAF sensor ignition feed circuit. Repair the wiring or replace any components as necessary. Did you complete the repair? If yes, go to step 16 . If no, go to step 13 .
11. Repair the open MAF sensor ground circuit. After repairs, go to step 16 .
12. Inspect the MAF sensor connector for proper terminal contact. Did you find and correct the condition? If yes, go to step 16 . If no, go to next step.
13. Replace the MAF/IAT sensor. See «MASS AIRFLOW SENSOR»(ref-150199-S12133959972002110600000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 16 .
14. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 16 .
15. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
16. Use the scan tool in order to clear the DTCs. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
17. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

Power-Up Test

1. The engine is operating on alternative fuel.
2. The battery voltage is more than 10 volts.

Cranking Test

1. The engine is cranking on alternative fuel, but has not yet started.
2. The battery voltage is more than 8 volts.

Running Test

1. The engine is operating on alternative fuel.
2. The engine has been operating for more than 10 seconds.
3. The battery voltage is more than 10 volts.
4. The engine speed is more than 500 RPM.

The AF ECM monitors a MAF sensor signal of more than 10,390 Hz for at least 0.5 second continuously.

The AF ECM will switch-over to gasoline fuel operation. Alternative fuel operation will not be attempted until the next ignition key cycle.

1. The AF ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
2. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «DIAGNOSTIC AIDS»(ref-151060-S34105765542002120600000) .
3. Start and idle the engine. Using the scan tool, establish communication with the PCM. Observe the MAF sensor data parameter. Does the MAF sensor data parameter indicate frequency more than 10.39 kHz? If yes, go to next step. If no, go to step 9 .
4. Turn OFF the ignition. Disconnect the MAF sensor connector. Start and idle the engine. Observe the MAF sensor data parameter with the engine idling. Does the MAF sensor data parameter indicate frequency more than 0 Hz? If yes, go to next step. If no, go to step 7 .
5. Turn OFF the ignition. Disconnect the AF ECM connectors. Start and idle the engine. Observe the MAF sensor data parameter with the engine idling. Does the MAF sensor data parameter indicate frequency more than 0 Hz? If yes, go to next step. If no, go to step 9 .
6. A high frequency signal may be induced on the MAF signal circuit by sources other than the MAF sensor. Inspect the vehicle for the following conditions: A MAF signal circuit routed near secondary ignition components. A MAF signal circuit routed near high current components such as motors, relays, etc. High power transceivers operating in or near the vehicle. Route the wiring harness correctly or remove the source of high frequency. Was an induced high frequency source found and corrected? If yes, go to step 10 . If no, go to step 8 .
7. Replace the MAF/IAT sensor. See «MASS AIRFLOW SENSOR»(ref-150199-S12133959972002110600000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 10 .
8. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 10 .
9. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
10. Use the scan tool in order to clear the DTCs. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
11. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

Inspect for the following conditions

1. A mis-routed harness. Inspect the MAF sensor harness for routing that is too close to the following components: Secondary ignition wires or coils. Solenoids. Relays. Motors.
2. The throttle plate for sticking, or for deposits on the throttle plate, or in the throttle bore.
3. Any un-metered air that enters the engine may cause this DTC to set. Make sure that the oil dip stick is seated, and the oil fill-cap is secure.

For an intermittent condition, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.

1. The engine is operating on alternative fuel.
2. The engine has been operating for greater than 5 seconds.
3. No AF ECM DTCs are set. DTCs P1215, P1432 and P1433 may be set.
4. TP sensor angle is less than 70 percent.

The AF ECM monitors that the MAP sensor pressure changes less than 6 kPa when the TP sensor angle changes by greater than 20 percent.

1. The AF ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
2. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ignition on. Using scan tool, establish communication with AF ECM. Observe and record MAP sensor kPa display. MAP sensor kPa reading should be 75 kPa at altitude or 95 kPa at sea level. Is the value greater than the value shown? If yes, go to next step. If no, go to step 8 .
3. Remove vacuum line from MAP sensor. Connect Hand Vacuum Pump (J-23738-A) to MAP sensor. Slowly apply vacuum one in. Hg at a time. Monitor MAP sensor kPa display on scan tool. Each one inch of vacuum should result in a 3 to 4 kPa drop in MAP sensor kPa reading. Observe MAP sensor kPa reading for a skip or jump while increasing vacuum up to 20 in. Hg. Did you observe a skip or jump? If yes, go to step 8 . If no, go to next step.
4. Monitor MAP sensor kPa reading with 20 in. Hg of vacuum applied. Is MAP sensor kPa reading less than 34 kPa? If yes, go to next step. If no, go to step 8 .
5. Disconnect MAP sensor from hand vacuum pump. The MAP sensor display should return to the value recorded in step 2 . Does the MAP sensor display return to the recorded value? If yes, go to next step. If no, go to step 8 .
6. Turn ON the ignition, with the engine OFF. Observe the TP sensor voltage parameter with a scan tool. Press the accelerator pedal from a closed throttle position, to a wide open throttle position. Does the TP sensor voltage parameter go from below 1.0 V to above 4.0 V? If yes, go to next step. If no, see «DTC P0121: TP SENSOR ANGLE OUT OF RANGE»(ref-151060-S03695337972002120600000) .
7. Fault is not present at this time. Are any DTCs stored that have not been diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, diagnose by symptom. See «TROUBLE SHOOTING - NO CODES - TRUCKS»(ref-149956) .
8. Inspect for proper terminal contact. Inspect the MAP sensor ground circuit for an open or high resistance between the MAP sensor connector and the PCM connector terminal. Repair connections or circuit as necessary. Did you find and correct the condition? If yes, go to step 10 . If no, go to next step.
9. Replace the MAP sensor. See «MANIFOLD ABSOLUTE PRESSURE SENSOR»(ref-150199-S32025807262002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
10. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
11. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The engine speed is greater than 500 RPM.
3. DTCs P0121, P0122 or P0123 are not set.
4. The TP angle is less than 4 percent when engine speed is less than 1200 RPM or TP angle is greater than 20 percent when engine speed is greater than 1300 RPM.

The AF ECM monitors a MAP sensor signal voltage of less than 0.098 volt for at least 4 seconds continuously.

The AF ECM will switchover to gasoline fuel operation.

The AF ECM will not attempt alternative fuel operation until the next key cycle.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn OFF the ignition for 30 seconds. Turn ON the ignition, with the engine OFF. With a scan tool, establish communication with the AF ECM. Observe and record the MAP Sensor data parameters. Compare the MAP sensor data display with the MAP sensor values from a known-good vehicle. Do both vehicles display the same MAP sensor data values? If yes, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM. If no, go to next step.
3. With a scan tool, establish communication with the PCM. Observe the MAP Sensor data parameter. Does the MAP sensor data parameter indicate MAP voltage less than 0.2 V? If yes, go to next step. If no, go to step 10 .
4. Disconnect the MAP sensor connector. Jumper the 5-volt reference circuit terminal to the MAP signal circuit terminal using a fused jumper. Observe the MAP Sensor data parameter. Does the MAP sensor data parameter indicate MAP voltage greater than 4.8 V? If yes, go to step 12 . If no, go to next step.
5. Remove the jumper. Connect the DMM between the 5-volt reference circuit terminal and a good ground. Measure the voltage on the 5-volt reference circuit. Is the voltage less than 4.8 V? If yes, go to step 8 . If no, go to next step.
6. Inspect the MAP sensor signal circuit for an open or high resistance between the sensor connector and the PCM. Inspect for proper terminal contact at both the PCM connector and the MAP sensor connector. Repair wiring or terminal contact as necessary. Did you find and correct the condition? If yes, go to step 15 . If no, go to next step.
7. Inspect the MAP sensor signal circuit for a short to engine ground or to the sensor ground circuit. Disconnect the AF ECM connector containing the MAP sensor signal circuit in order to determine if the AF ECM is the cause of the short. Repair wiring as necessary. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 13 .
8. Inspect the 5-volt reference circuit for an open or high resistance condition between the sensor connector and the PCM connector terminal. Inspect for proper terminal contact at both the PCM connector and the MAP sensor connector. Repair wiring or terminal contact as necessary. Did you find and correct the condition? If yes, go to step 15 . If no, go to next step.
9. Inspect all 5-volt reference circuits for a short to ground. Some 5-volt reference circuits are shared within the PCM. The short may be located on either of the 5-volt reference circuits that the PCM provides. Disconnect the AF ECM connector that contains the 5-volt reference circuit in order to determine if the AF ECM is the cause of the short. Disconnect the other sensors that are connected to the PCM 5-volt reference circuits in order to determine if a sensor is the cause of the short. Repair wiring or replace components as necessary. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 13 .
10. Test the MAP sensor signal circuit for an open between the AF ECM and the splice. Did you find and correct the condition? If yes, go to step 15 . If no, go to next step.
11. Inspect for proper terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 14 .
12. Replace the MAP sensor. See «MANIFOLD ABSOLUTE PRESSURE SENSOR»(ref-150199-S32025807262002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 15 .
13. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 15 .
14. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
15. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
16. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. See INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM. If any additional DTCs were stored, proceed to the applicable DTC. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .

1. The engine is operating on alternative fuel.
2. The engine speed is greater than 500 RPM.
3. DTCs P0121, P0122, P0123 are not set.
4. The TP angle is less than 4 percent.

The AF ECM monitors a MAP sensor signal voltage of greater than 4.8 volts for at least 4 seconds continuously.

The AF ECM will switchover to gasoline fuel operation.

The AF ECM will not attempt alternative fuel operation until the next key cycle.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
3. Turn ON the ignition, with the engine OFF. With a scan tool, establish communication with the PCM. Observe the MAP sensor data parameter. Does the MAP sensor data parameter indicate MAP voltage greater than 4.8 V? If yes, go to next step. If no, go to step 12 .
4. Disconnect the MAP Sensor connector. Observe the MAP Sensor data parameter with a scan tool. Does the MAP sensor data parameter indicate MAP voltage less than 0.25 V? If yes, go to next step. If no, go to step 9 .
5. Probe the ground circuit of the MAP sensor with a test lamp connected to battery voltage. Does the test lamp illuminate? If yes, go to next step. If no, go to step 8 .
6. Measure the voltage on the 5-volt reference circuit with a DMM. Does the DMM display a voltage less than 5.2 V? If yes, go to step 10 . If no, go to next step.
7. Test all 5-volt reference circuit for a short to voltage. Some 5-volt reference circuits are shared within the PCM. The short may be located on either of the 5-volt reference circuits that the PCM provides. Disconnect the AF ECM connector that contains the 5-volt reference circuit in order to determine if the AF ECM is the cause of the short to voltage. Repair wiring as necessary. Did you find and correct the condition? If yes, go to step 13 . If no, go to step 11 .
8. Test the sensor ground circuit for an open or high resistance condition between the sensor connector and the PCM connector terminal. Inspect for proper terminal contact at both the PCM connector and the MAP sensor connector. Repair the wiring or terminal contact as necessary. Did you find and correct the condition? If yes, go to step 13 . If no, go to step 11 .
9. Test the MAP signal circuit for a short to voltage. Disconnect the AF ECM connector that contains the MAP signal circuit in order to determine if the ECM is the cause of the short to voltage. Did you find and correct the condition? If yes, go to step 13 . If no, go to step 11 .
10. Replace the MAP sensor. See «MANIFOLD ABSOLUTE PRESSURE SENSOR»(ref-150199-S32025807262002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 13 .
11. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 13 .
12. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
13. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
14. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The engine has been operating for greater than 1.5 minutes.
3. The vehicle speed is greater than 1 MPH.

The AF ECM monitors an IAT sensor signal voltage of less than 0.019 volt for at least 5 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
3. Using scan tool, communicate with PCM. Observe IAT sensor temperature. Does the IAT sensor data parameter indicate IAT temperature warmer than 280°F (138°C)? If yes, go to next step. If no, go to step 7 .
4. Disconnect the MAF/IAT sensor. Does the IAT sensor data parameter indicate IAT temperature warmer than 280°F (138°C)? If yes, go to next step. If no, go to step 8 .
5. Turn ignition off. Disconnect the PCM connector that contains the IAT signal circuit. Disconnect the AF ECM connector that contains the IAT signal circuit. With the MAF/IAT sensor connector disconnected, test the IAT signal circuit for continuity to engine ground and also to the sensor ground circuit. If a shorted to ground IAT signal circuit is indicated repair the circuit as necessary. Was a IAT signal circuit repair made? If yes, go to step 11 . If no, go to next step.
6. Connect the PCM connector C2. Leave the AF ECM connector disconnected. Turn the ignition ON, with the engine OFF. With the MAF/IAT sensor connector disconnected, observe the PCM IAT Sensor data parameter. Does the IAT sensor data parameter indicate IAT temperature warmer than 280°F (138°C)? If yes, go to step 9 . If no, go to step 10 .
7. Inspect the signal circuit between the circuit splice and the PCM connector for the following conditions: An open. High resistance. Poor terminal contact. Repair wiring or terminals as necessary. Did you find and correct the condition? If yes, go to step 11 . If no, go to step 10 .
8. Replace the MAF/IAT sensor. See «MASS AIRFLOW SENSOR»(ref-150199-S12133959972002110600000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 11 .
9. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 11 .
10. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
11. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
12. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The engine has been operating for greater than 1.5 minutes when the engine start-up temperature is greater than 32°F (0°C).
3. The engine has been operating for greater than 10 minutes when the engine start-up temperature is 32°F (0°C) or less.
4. The vehicle speed is less than 1 MPH for at least 10 seconds continuously.
5. Calculated air flow is less than 250 grams per second for at least 10 seconds continuously.

The AF ECM monitors an IAT sensor signal voltage greater than 4.80 volts for at least 5 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
3. With a scan tool, establish communication with the Powertrain Control Module (PCM). Observe the IAT Sensor data parameter. Does the IAT sensor data parameter indicate IAT temperature colder than -27°F (-34°C)? If yes, go to next step. If no, go to step 12 .
4. Disconnect the MAF/IAT sensor. Connect a 3-amp fused jumper between the IAT sensor connector terminals. If the fuse of the jumper opens, repair the short to voltage on the IAT sensor signal circuit. If a short to voltage condition is observed, inspect the sensor for an open condition after the wiring repair is completed. Does the IAT sensor data parameter indicate IAT temperature warmer than 298°F (148°C)? If yes, go to step 7 . If no, go to next step.
5. Connect the fused jumper between the IAT signal circuit and a known-good ground. Does the IAT sensor data parameter indicate IAT temperature warmer than 298°F (148°C)? If yes, go to step 9 . If no, go to next step.
6. Turn the ignition OFF. Disconnect the PCM connector C2. Test the IAT signal circuit for continuity with a DMM. Does the DMM indicate continuity? If yes, go to step 11 . If no, go to step 10 .
7. Inspect for poor connections at the harness connector of the MAF/IAT sensor. Did you find and correct the condition? If yes, go to step 17 . If no, go to next step.
8. Replace the MAF/IAT sensor. See «MASS AIRFLOW SENSOR»(ref-150199-S12133959972002110600000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 17 .
9. Repair the open sensor ground circuit between the IAT sensor connector and the PCM. After repairs, go to step 17 .
10. Repair the open IAT signal circuit between the IAT sensor connector and the PCM. After repairs, go to step 17 .
11. Inspect for proper terminal contact at the PCM connector. Did you find and correct the condition? If yes, go to step 17 . If no, go to step 15 .
12. Turn the ignition OFF. Disconnect the AF ECM connector C002. With the MAF/IAT sensor connector disconnected, test the IAT signal circuit for continuity. Is IAT signal circuit continuity indicated? If yes, go to next step. If no, go to step 14 .
13. Inspect for proper terminal contact at the AF ECM connector. Did you find and correct the condition? If yes, go to step 17 . If no, go to step 16 .
14. Repair the open IAT signal circuit between the IAT signal circuit splice and the AF ECM. After repairs, go to step 17 .
15. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 17 .
16. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
17. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
18. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The engine has been operating for greater than 5 seconds.

The AF ECM monitors an ECT sensor signal voltage of less than 0.019 volt for at least 5 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
3. Using scan tool, establish communication with the Powertrain Control Module (PCM). Observe the ECT sensor data parameter. Does the ECT sensor data parameter indicate ECT temperature warmer than 284°F (140°C)? If yes, go to step 4 . If no, go to step 7 .
4. Disconnect the ECT sensor. Does the ECT sensor data parameter indicate ECT temperature warmer 284°F (140°C)? If yes, go to next step. If no, go to step 8 .
5. Turn ignition off. Disconnect the PCM connector that contains the ECT signal circuit. Disconnect the AF ECM connector that contains the ECT signal circuit. Test the ECT signal circuit for continuity to engine ground and also to the sensor ground circuit. If a short to ground ECT signal circuit is indicated repair the circuit as necessary. Was a ECT signal circuit repair made? If yes, go to step 11 . If no, go to next step.
6. Connect the PCM connector that contains the ECT signal circuit. Leave the AF ECM connector disconnected. Turn ignition on, with engine off. With the ECT sensor connector disconnected observe the PCM ECT sensor data parameter. Does the ECT sensor data parameter indicate ECT temperature warmer than 284°F (140°C)? If yes, go to step 9 . If no, go to step 10 .
7. Inspect the signal circuit between the circuit splice and PCM connector for an open circuit, high resistance or poor terminal contact. Repair wiring or terminals as necessary. Was a wiring or terminal repair made? If yes, go to step 11 . If no, go to step 10 .
8. Replace the ECT sensor. See «ENGINE COOLANT TEMPERATURE SENSOR»(ref-150199-S26732808882002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 11 .
9. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 11 .
10. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
11. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
12. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The engine has been operating for greater than 5 seconds when the Intake Air Temperature is greater than 32°F (0°C) or engine has been operating for greater than 10 minutes when the Intake Air Temperature is less than 32°F (0°C).

The AF ECM monitors an ECT sensor signal voltage of greater than 4.8 volts for at least 5 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
3. With a scan tool, establish communication with the PCM. Observe the ECT sensor data parameter. Does the ECT sensor data parameter indicate ECT temperature colder than -27°F (-34°C)? If yes, go to next step. If no, go to step 7 .
4. Disconnect the ECT sensor. Connect a 3-amp fused jumper between the ECT sensor connector terminals. If the fuse of the jumper opens, repair the short to voltage on the ECT sensor signal circuit. If a short to voltage condition is observed, inspect the sensor for an open condition after the wiring repair is completed. Does the ECT sensor data parameter indicate ECT temperature warmer than 284°F (140°C)? If yes, go to step 8 . If no, go to next step.
5. Connect the fused jumper between the ECT signal circuit and a known-good ground. Does the ECT sensor data parameter indicate ECT temperature warmer than 284°F (140°C)? If yes, go to step 11 . If no, go to next step.
6. Turn the ignition OFF. Disconnect the PCM connector C1. Test the ECT signal circuit for continuity with a DMM. Does the DMM display continuity? If yes, go to step 9 . If no, go to step 12 .
7. Turn the ignition OFF. Disconnect the AF ECM connector C002. Test the ECT signal circuit for continuity with a DMM. Does the DMM display continuity? If yes, go to step 10 . If no, go to step 13 .
8. Inspect for proper terminal contact at the harness connector of the ECT sensor. Did you find and correct the condition? If yes, go to step 17 . If no, go to step 14 .
9. Inspect for proper terminal contact at the harness connector of the PCM. Did you find and correct the condition? If yes, go to step 17 . If no, go to step 15 .
10. Inspect for proper terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 17 . If no, go to step 16 .
11. Repair the open sensor ground circuit between the ECT sensor connector and the PCM. After repairs, go to step 17 .
12. Repair the open ECT signal circuit between the ECT sensor connector and the PCM. After repairs, go to step 17 .
13. Repair the open ECT signal circuit between the ECT signal circuit splice and the AF ECM. After repairs, go to step 17 .
14. Replace the ECT sensor. See «ENGINE COOLANT TEMPERATURE SENSOR»(ref-150199-S26732808882002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 17 .
15. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 17 .
16. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
17. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
18. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The engine has been operating for greater than 2 seconds.
3. No AF ECM DTCs are set. DTCs P1215, P1432 and P1433 may be set.
4. The engine MAP is less than 45 kPa.
5. The TP sensor angle has changed less than 2 percent for 2 seconds continuously.

The TP sensor angle is greater than the specified percentage at the given engine speed. See TP ANGLE-VS-ENGINE SPEED SPECIFICATIONS table.

TP ANGLE-VS-ENGINE SPEED SPECIFICATIONS

1. The AF ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
2. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. With a scan tool, establish communications with the Powertrain Control Module (PCM). Observe the MAP sensor voltage parameter. Start the engine. Does the MAP sensor voltage parameter change value after you started the engine? If yes, go to next step. If no, see «DTC P0106: MAP/TP SENSOR PERFORMANCE/RANGE»(ref-151060-S04390065212002120600000) .
3. Turn OFF the engine. Turn ON the ignition, with the engine OFF. Observe the TP sensor voltage parameter while moving the accelerator pedal from a closed throttle to a wide open throttle. Does the TP sensor voltage go from below 1.0 V to above 4.0 V? If yes, go to next step. If no, go to step 7 .
4. With a scan tool, establish communications with the AF ECM. Observe the MAP sensor voltage parameter. Start the engine. Does the MAP sensor voltage parameter change value after you started the engine? If yes, go to next step. If no, see «DTC P0106: MAP/TP SENSOR PERFORMANCE/RANGE»(ref-151060-S04390065212002120600000) .
5. Turn OFF the engine. Turn ON the ignition, with the engine OFF. Observe the TP sensor voltage parameter while moving the accelerator pedal from a closed throttle to a wide open throttle. Does the TP sensor voltage go from below 1.0 V to above 4.0 V? If yes, go to next step. If no, go to step 14 .
6. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
7. Disconnect the TP sensor. Observe the TP sensor voltage parameter with a scan tool. Is the TP sensor voltage below 0.75 V? If yes, go to step 9 . If no, go to next step.
8. Turn OFF the ignition. Disconnect the AF ECM. Turn ON the ignition, with the engine OFF. Observe the TP sensor voltage parameter with a scan tool. Is the TP sensor voltage below 0.75 V? If yes, go to step 12 . If no, go to step 13 .
9. Connect a 3-amp fused jumper wire between the 5-volt reference circuit of the TP sensor and the signal circuit of the TP sensor. Is the TP sensor voltage near 5 V? If yes, go to next step. If no, go to step 15 .
10. Probe the TP sensor ground circuit with a test lamp connected to battery voltage. Does test lamp illuminate? If yes, go to step 16 . If no, go to next step.
11. Test the TP sensor ground circuit for an open, or for high resistance. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 17 .
12. Test the TP sensor signal circuit for a short to voltage between the AF ECM and the splice. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 21 .
13. Test the TP sensor signal circuit for a short to voltage. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 17 .
14. Test the TP sensor signal circuit for high resistance between the AF ECM and the splice. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 18 .
15. Test the TP sensor signal circuit and the TP sensor 5-volt reference circuit for an open, or for high resistance. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 17 .
16. Test for an intermittent and for a poor connection at the TP sensor. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 19 .
17. Test for an intermittent and for a poor connection at the PCM. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 20 .
18. Test for an intermittent and for a poor connection at the AF ECM. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 21 .
19. Replace the TP sensor. See «THROTTLE POSITION SENSOR»(ref-150199-S25710483362002111800000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 22 .
20. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 22 .
21. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
22. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
23. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The engine is operating on alternative fuel.

The AF ECM monitors a TP sensor signal voltage of less than 0.09 volt for at least 1 second continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to step 4 . If no, go to next step.
3. Open and close the throttle while observing the TP Sensor data parameter. Does the TP sensor data parameter ever indicate TP voltage less than 0.09 V? If yes, go to step 13 . If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Turn ON the ignition, with the engine OFF. With a scan tool, establish communication with the PCM. Observe the TP Sensor data parameter. Does the TP sensor data parameter indicate TP voltage less than 0.09 V? If yes, go to next step. If no, go to step 9 .
5. Disconnect the TP sensor connector. Jumper the 5-volt reference circuit terminal to the TP signal circuit terminal with a 3-amp fused jumper. With a scan tool, observe the TP Sensor data parameter. Does the TP sensor data parameter indicate TP voltage greater than 4.8 V? If yes, go to step 13 . If no, go to next step.
6. Remove the jumper. Measure the voltage on the 5-volt reference circuit with a DMM. Does the DMM display a voltage less than 4.8 V? If yes, go to step 11 . If no, go to next step.
7. Inspect the TP sensor signal circuit for an open or high resistance between the sensor connector and the PCM. Inspect for proper terminal contact at both the PCM connector and the TP sensor connector. Did you find and correct the condition? If yes, go to step 12 . If no, go to next step.
8. Test the TP sensor signal circuit for a short to engine ground or to the sensor ground circuit. Disconnect the AF ECM connector containing the TP sensor signal circuit in order to determine if the ECM is the cause of the short. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 14 .
9. Test the signal circuit of the TP sensor for an open between the AF ECM and the splice. Did you find and correct the condition? If yes, go to step 16 . If no, go to next step.
10. Inspect for proper connections at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 15 .
11. Inspect the 5-volt reference circuit for an open or high resistance condition between the sensor connector and the PCM. Inspect for proper terminal contact at both the PCM connector and the TP sensor connector. Did you find and correct the condition? If yes, go to step 16 . If no, go to next step.
12. Inspect all 5-volt reference circuits for a short to ground. Some 5-volt reference circuits are shared within the PCM. The short may be located on either of the 5-volt reference circuits that the PCM provides. Disconnect the AF ECM connector that contains the 5-volt reference circuit in order to determine if the ECM is the cause of the short. Disconnect the other sensors that are connected to the PCM 5-volt reference circuits in order to determine if a sensor is the cause of the short. Repair wiring or replace components as necessary. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 14 .
13. Replace the TP sensor. See «THROTTLE POSITION SENSOR»(ref-150199-S25710483362002111800000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 16 .
14. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 16 .
15. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
16. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
17. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The engine is operating on alternative fuel.

The AF ECM monitors a TP sensor signal voltage of greater than 4.9 volts for at least 1 second continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to step 4 . If no, go to next step.
3. With a scan tool, establish communication with the PCM. Open and close the throttle while observing the TP Sensor data parameter. Does the TP sensor data parameter ever indicate TP voltage greater than 4.9 V? If yes, go to step 11 . If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Turn the ignition ON, with the engine OFF. With a scan tool, establish communication with the PCM. Observe the TP Sensor data parameter. Fully depress the accelerator pedal. Does the TP sensor data parameter indicate TP voltage greater than 4.9 V? If yes, go to next step. If yes, go to step 13 .
5. Disconnect the TP Sensor connector. Observe the TP Sensor data parameter. Does the TP sensor data parameter indicate TP voltage less than 0.75 V? If yes, go to next step. If no, go to step 10 .
6. Probe the TP sensor ground circuit with a test lamp connected to battery voltage. Does the test lamp illuminate? If yes, go to next step. If no, go to step 9 .
7. Measure the voltage on the 5-volt reference circuit of the TP sensor with a DMM. Is the voltage less than 5.2 V? If yes, go to step 11 . If no, go to next step.
8. Inspect all 5-volt reference circuit for a short to voltage. Some 5-volt reference circuits are shared within the PCM. The short may be located on any of the 5-volt reference circuits that the PCM provides. Disconnect the AF ECM connector that contains the 5-volt reference circuit in order to determine if the AF ECM is the cause of the short to voltage. Repair wiring as necessary. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 12 .
9. Test the sensor ground circuit for an open or high resistance condition between the sensor connector and the PCM connector terminal. Inspect for proper terminal contact at both the PCM connector and the TP sensor connector. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 12 .
10. Test the TP signal circuit for a short to voltage. Disconnect the AF ECM connector that contains the TP signal circuit in order to determine if the ECM is the cause of the short to voltage. Repair wiring as necessary. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 12 .
11. Replace the TP sensor. See «THROTTLE POSITION SENSOR»(ref-150199-S25710483362002111800000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 14 .
12. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 14 .
13. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
14. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
15. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. AF ECM DTCs P0112, P0113, P0117, P0118 are not set.
3. This DTC has not tested this ignition cycle.
4. The engine has been operating for greater than 8.5 minutes.
5. The vehicle speed is greater than 5 MPH.
6. The desired gas flow is greater than 0.5 gram per second.
7. The Intake Air Temperature is greater than 5°F (-15°C).
8. The engine coolant temperature at engine start-up is 1-91°F (-17-33°C).

The AF ECM monitors an engine coolant temperature that has NOT increased greater than the start-up temperature by 77°F (25°C).

1. The AF ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
2. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Verify proper cooling system and thermostat operation. Was a cooling system repair made? If yes, go to step 10 . If no, go to next step.
3. Turn ON the ignition, with the engine OFF. Using scan tool, establish communication with the AF ECM. Disconnect the ECT sensor. Additional DTCs may set. Clear DTCs when diagnosis is complete. Observe the ECT sensor data parameter. Does the ECT sensor data parameter indicate ECT voltage greater than 4.8 volts? If yes, go to next step. If no, go to step 6 .
4. Connect a fused jumper between the ECT sensor connector signal and ground circuit terminals. Additional DTCs may set. Clear DTCs when diagnosis is complete. Does the ECT sensor data parameter indicate ECT voltage less than 0.019 volt? If yes, go to next step. If no, go to step 7 .
5. Reconnect the ECT sensor connector. Install a thermometer in the engine cooling system in order to measure actual coolant temperature. Start and idle engine. Allow engine to operate until ECT sensor data parameter no longer increases (temperature has stabilized). Observe ECT sensor data parameter. Measure the actual engine coolant temperature. Compare the ECT sensor data parameter to actual engine coolant temperature. Does the ECT sensor data parameter indicate a temperature near the actual engine coolant temperature? If yes, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM. If no, go to step 8 .
6. Inspect the ECT signal circuit for a shorted or low resistance to ground condition. Repair wiring or terminals as necessary. Was a wiring or terminal repair made? If yes, go to step 10 . If no, go to step 9 .
7. Inspect the ECT signal and ground circuits for an open or high resistance condition. Inspect the PCM connector, the AF ECM connector and the ECT sensor connector for proper terminal contact. Repair wiring or terminals as necessary. Was a wiring or terminal repair made? If yes, go to step 10 . If no, go to step 9 .
8. Replace ECT sensor. See «ENGINE COOLANT TEMPERATURE SENSOR»(ref-150199-S26732808882002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 10 .
9. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
10. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
11. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The most likely cause of this DTC is an improperly operating engine coolant system and thermostat. Diagnose thermostat to verify proper cooling system and thermostat operation.

A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. See INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM. If any additional DTCs were stored, proceed to the applicable DTC. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .

1. The engine is operating on alternative fuel.
2. Engine speed is greater than 400 RPM.
3. Engine coolant temperature is greater than 160°F (70°C).
4. The AF ECM determines the HO2S has been at operating temperature for at least 30 seconds. The engine coolant temperature must be greater than 104°F (40°C) and the desired gas flow is greater than 0.3 gram per second for 30 seconds.
5. The AF ECM is operating in Closed Loop fuel control.

The AF ECM monitors an HO2S signal voltage of less than 0.08 volt (80 mV) for 15 seconds continuously.

The AF ECM will control fuel trim based upon the Bank 2 Sensor 1 signal. If a Bank 2 Sensor 1 DTC sets, the AF ECM will operate in Open Loop fuel control.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Run the engine until normal operating temperature is reached. Using scan tool, establish communication with the AF ECM. Using scan tool, monitor the AF ECM HO2S voltage for the sensor that applies to this DTC. Is the HO2S voltage fixed less than 78 mV? If yes, go to step 4 . If no, go to next step.
3. Condition necessary to set DTC is not present at this time. Check for the following conditions: Signal wire intermittently shorted to engine ground or sensor low circuit. Open between engine harness circuit splice and AF ECM connectors. HO2S connector water intrusion. Exhaust leak between HO2S and engine. Intermittent lean engine condition such as vacuum leaks, incorrect or defective PCV valve or system, incorrect fuel pressure or an inaccurate MAF sensor. Was a condition found and repaired? If yes, go to step 15 . If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Run the engine until normal operating temperature is reached. Using scan tool, establish communication with the Powertrain Control Module (PCM). Using scan tool, monitor the HO2S voltage for the sensor that applies to this DTC. Is the HO2S voltage fixed less than 78 mV? If yes, go to next step. If no, go to step 10 .
5. Turn ignition off. Disconnect AF ECM connectors. Start engine and run until operating temperature is reached. Using scan tool, monitor suspect HO2S voltage. Is the HO2S voltage fixed less than 78 mV? If yes, go to next step. If no, go to step 11 .
6. Turn ignition off. Disconnect the HO2S connector for the sensor that applies to this DTC. Leave the AF ECM disconnected. Jumper the HO2S low circuit terminal, engine harness side, to a known-good ground. Turn ON the ignition, with the engine OFF. Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC. Is the HO2S voltage within 351-551 mV? If yes, go to next step. If no, go to step 8 .
7. HO2S my be detecting a lean condition. Check for an exhaust leak between HO2S and engine, vacuum leaks, water intrusion into HO2S connector or incorrect, a defective PCV valve or system or an inaccurate MAF sensor. Repair any or similar engine conditions as necessary. Was a condition found and repaired? If yes, go to step 15 . If no, go to step 13 .
8. Turn ignition off. Remove the jumper from the HO2S low circuit terminal. Disconnect PCM connector that contains the HO2S high signal circuit. Using the Digital Multimeter (DMM), measure and record the resistance between the HO2S high signal circuit and ground. Measure and record the resistance between the HO2S high signal circuit and low circuit. Is infinite resistance measured on both tests? If yes, go to step 14 . If no, go to next step.
9. Repair HO2S signal high circuit for short to ground or short to HO2S low circuit. After repairs, go to step 15 .
10. Test the HO2S high and low circuits for continuity between the engine harness circuit splices and the AF ECM connector terminals. Measure for continuity between the oxygen sensor connector and the AF ECM connector. Test for proper terminal contact at the AF ECM connector terminals and at the in-line engine harness connector terminals. Was a wiring or terminal repair made? If yes, go to step 15 . If no, go to step 12 .
11. HO2S may only be detecting lean condition during alternate fuel (CNG) operation. Perform fuel system diagnosis. See under «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article. Repair as necessary. Was a lean alternative fuels system condition found and repaired? If yes, go to step 15 . If no, go to next step.
12. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to step 15 .
13. Replace suspect HO2S. See «OXYGEN SENSORS»(ref-150199-S40719737762002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 15 .
14. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
15. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
16. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. Engine speed is greater than 400 RPM.
3. Engine coolant temperature is greater than 160°F (70°C).
4. The AF ECM determines the HO2S has been at operating temperature for at least 30 seconds. The engine coolant temperature must be greater than 104°F (40°C) and the desired gas flow is greater than 0.3 gram per second for 30 seconds.
5. The AF ECM is operating in Closed Loop fuel control.

The AF ECM monitors an HO2S signal voltage of greater than 1 volt (1000 mV) for 3 seconds continuously.

The AF ECM will Control fuel trim based upon the Bank 2 Sensor 1 signal. If a Bank 2 Sensor 1 DTC sets the AF ECM will operate in Open Loop fuel control.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Run the engine until normal operating temperature is reached. Using scan tool, establish communication with the AF ECM. Using scan tool, monitor the AF ECM HO2S voltage for the sensor that applies to this DTC. Is the HO2S voltage fixed greater than 800 mV? If yes, go to step 5 . If no, go to next step.
3. Condition necessary to set DTC is not present at this time. Check for the following conditions: Signal wire intermittently shorted to voltage. HO2S connector water intrusion. Silicone contaminated HO2S. An intermittent rich engine condition such as EVAP canister purge system malfunction, gasoline contaminated engine oil, incorrect fuel pressure, leaking gasoline fuel pressure regulator or an inaccurate MAF sensor. Repair as necessary. Was a condition found and repaired? If yes, go to step 15 . If no, go to next step.
4. The fault is not present. See «DIAGNOSTIC AIDS»(ref-151060-S21000578562002120600000) . Are any DTCs stored that have not been diagnosed? If yes, go to «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
5. Run the engine until normal operating temperature is reached. Using scan tool, establish communication with the Powertrain Control Module (PCM). Using scan tool, monitor the HO2S voltage for the sensor that applies to this DTC. Is the HO2S voltage fixed greater than 800 mV? If yes, go to next step. If no, go to step 12 .
6. Turn ignition off. Disconnect the AF ECM connectors. Turn ON the ignition, with the engine OFF. Using scan tool, monitor the HO2S voltage for the sensor that applies to this DTC. Is the HO2S voltage fixed greater than 800 mV? If yes, go to next step. If no, go to step 11 .
7. Turn ignition off. Disconnect the HO2S connector for the sensor that applies to this DTC. Leave the AF ECM disconnected. Jumper the HO2S low circuit terminal, engine harness side to a known-good ground. Turn the ignition ON, with the engine OFF. Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC. Is the HO2S voltage within 351-551 mV? If yes, go to next step. If no, go to step 9 .
8. The HO2S is detecting a rich exhaust condition or may be contaminated. Inspect for water intrusion in HO2S water intrusion, silicone contaminated HO2S, fuel contaminated engine oil, EVAP canister purge system malfunction, incorrect fuel pressure, a leaking gasoline fuel pressure regulator or an inaccurate MAF sensor. Repair any of the conditions as necessary. Was a condition found and repaired? If yes, go to step 15 . If no, go to step 13 .
9. Turn ignition off. Remove the jumper from the HO2S low circuit terminal. Disconnect the HO2S high signal circuit from the PCM connector. Turn ignition on, with engine off. Using the Digital Multimeter (DMM) measure the voltage between the HO2S high signal circuit terminal (harness side) and ground. Is the voltage measured greater than 20 mV? If yes, go to next step. If no, go to step 14 .
10. Repair the HO2S signal high circuit shorted to voltage. After repairs, go to step 15 .
11. HO2S may only be detecting lean condition during alternate fuel (CNG) operation. Perform fuel system diagnosis. See under «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article. Repair as necessary. Was a lean alternative fuels system condition found and repaired? If yes, go to step 15 . If no, go to next step.
12. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to step 15 .
13. Replace suspect HO2S. See «OXYGEN SENSORS»(ref-150199-S40719737762002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 15 .
14. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
15. Reconnect all disconnected or removed components, connectors, fuses, etc. Using scan tool, clear all DTC information from BOTH the Powertrain Control Module (PCM) and the AF ECM. Using scan tool, communicate with the BOTH the PCM and the AF ECM in order to reset the Fuel Trim values to zero. Start engine. Operate the vehicle within the Conditions For Running DTC until the scan tool indicates the diagnostic ran. Does the scan tool indicate the DTC passed? If yes, system is okay. If no, go to step 2 .

A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. See INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM. If any additional DTCs were stored, proceed to the applicable DTC. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .

1. The engine is operating on alternative fuel.
2. The engine speed is more than 400 RPM.
3. The engine has been operating for at least 4 minutes.
4. The AF ECM determines the HO2S has been at operating temperature for at least 30 seconds. Operating temperature is indicated when engine coolant temperature is more than 104°F (40°C) and desired gas flow is greater than 0.3 gram per second for 30 seconds.

The AF ECM monitors an HO2S signal voltage stuck between 0.347 volt and 0.550 volt (inactive) for 9 seconds continuously; or the AF ECM monitors an HO2S signal voltage between 0.00 volts and 1.0 volt that has not performed a Rich/Lean switch for 15 seconds continuously.

The AF ECM will control fuel trim based upon the Bank 2 Sensor 1 signal. If a Bank 2 Sensor 1 DTC sets, the AF ECM will operate in Open Loop fuel control.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Is DTC P1215 also set? If yes, see «DTC P1215: GROUND FAULT DETECTION INDICATED»(ref-151060-S29314015772002120600000) . If no, go to next step.
3. Use a scan tool in order to establish communication with the AF ECM. Capture the DTC information from the AF ECM into the scan tool. Clear the DTC information using a scan tool. Reset the Fuel Trim values. Start and idle the engine until a normal operating temperature is reached. Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC. Does the voltage vary outside 300 mV-600 mV? If yes, go to next step. If no, go to step 5 .
4. The condition that set this DTC is not present. This DTC may have been set by one of the following conditions: Water intrusion of the HO2S connector. An intermittently open HO2S high signal circuit An intermittently open HO2S low circuit. An HO2S Low circuit with high resistance to ground. Any AF ECM ground circuits with high resistance to ground. An inoperative HO2S heater. A faulty HO2S or Control Module connector terminal contact. An exhaust system leak, typically within 12 inches up-stream of the HO2S. A defective HO2S. Repair any of the above conditions as necessary. Was a condition found and repaired? If yes, go to step 23 . If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
5. Use the scan tool in order to establish communication with the PCM. Start and idle the engine until a normal operating temperature is reached. Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC. Is the voltage fixed within 300 mV-600 mV? If yes, go to next step. If no, go to step 17 .
6. Turn the ignition OFF. Disconnect the HO2S connector for the sensor that applies to this DTC. Jumper the HO2S low circuit to a known-good ground. Jumper the HO2S high signal circuit to a known-good ground. Turn the ignition ON, with the engine OFF. Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC. Is the voltage less than 20 mV? If yes, go to step 9 . If no, go to next step.
7. Turn the ignition OFF. Remove the jumpers from the HO2S terminals. Disconnect the PCM connector containing the HO2S high and low circuits. Use a Digital Multimeter (DMM) in order to measure the continuity of the HO2S high signal circuit and the HO2S low circuit between the control module connector terminal and the HO2S connector terminal, engine harness side. Are both resistances less than 5 ohms? If yes, go to next step. If no, go to step 11 .
8. Inspect for proper PCM connector terminal contact. Repair the terminals as necessary. Was a terminal repair made? If yes, go to step 23 . If no, go to step 22 .
9. Remove the jumpers from the HO2S terminals. Connect the Unpowered Test Lamp (J 34142-B) between the HO2S heater ignition feed and heater ground circuit terminals, engine harness side. Turn the ignition ON, with the engine OFF. Does the test lamp illuminate? If yes, go to next step. If no, go to step 12 .
10. Turn the ignition OFF. Reconnect the HO2S connector. Disconnect the PCM connector containing the HO2S low circuit and the PCM connector containing the control module ground circuits. Use the DMM in order to measure the resistance between the HO2S low circuit terminal and one of the control module ground circuit terminals. Is the resistance less than 500 ohms? If yes, go to step 14 . If no, go to step 13 .
11. Repair the circuit that measured high resistance. After repairs, go to step 23 .
12. Repair the HO2S heater ignition or ground circuit. Probable causes include the following conditions: An open circuit. A high circuit resistance. A faulty ground connection. A blown heater fuse (other DTCs may be set). After repairs, go to step 23 .
13. Repair the high resistance between the HO2S low circuit and the control module ground circuit. Probable causes include the following conditions: A faulty HO2S connector terminal contact. An open sensor harness low circuit (requires HO2S replacement). A sensor harness low circuit with high resistance (requires HO2S replacement). A faulty PCM ground connection. A high PCM ground circuit resistance An ungrounded exhaust system. After repairs, go to step 23 .
14. Inspect for exhaust system leaks upstream of the suspect HO2S. Repair the exhaust as necessary. Was an exhaust system repair made? If yes, go to step 23 . If no, go to next step.
15. Allow engine to cool to ambient temperature. Use the DMM in order to re-measure the resistance between the HO2S low circuit terminal and a PCM ground circuit terminal. Is the resistance less than 500 ohms? If yes, go to step 18 . If no, go to next step.
16. Remove the HO2S from the exhaust. Clean the HO2S mounting threads. Apply anti-seize compound (GM P/N 5613695) or equivalent to the sensor threads. Tighten the HO2S to the proper torque. See «OXYGEN SENSORS»(ref-150199-S40719737762002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. Use the DMM in order to measure the resistance between the HO2S low circuit terminal and a PCM ground circuit terminal. Is the resistance less than 500 ohms? If yes, go to step 23 . If no, go to step 21 .
17. Inspect the oxygen sensor signal and low circuit for continuity between the AF ECM connector terminals and the oxygen sensor connector terminals. Inspect these circuits for shorts to other circuits or to ground. Ensure that the oxygen sensor, PCM and the AF ECM connectors are disconnected before testing the circuits. Repair the wiring as necessary. Was a wire repair made? If yes, go to step 23 . If no, go to step 20 .
18. With a scan tool establish communications with the AF ECM. Observe the Fuel Rail Pressure (FRP) sensor voltage parameter. Does the scan tool display a FRP sensor voltage less than 1 V? If yes, see «DTC P0192: FUEL RAIL PRESSURE SENSOR CIRCUIT - LOW VOLTAGE»(ref-151060-S02919902812002121000000) . If no, go to next step.
19. Perform fuel system diagnosis. See «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article. Did you find and correct a CNG fuel system condition? If yes, go to step 23 . If no, go to step 21 .
20. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to step 23 .
21. Replace suspect HO2S. See «OXYGEN SENSORS»(ref-150199-S40719737762002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 23 .
22. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
23. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
24. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. See INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM. If any additional DTCs were stored, proceed to the applicable DTC. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .

1. The engine is operating on alternative fuel.
2. No AF ECM ECT DTCs are set.
3. The start-up engine coolant temperature is less than 91°F (33°C).
4. The engine has been operating for at least 5 seconds.
5. The average mass air flow is less than 35 grams per second.
6. The battery voltage is more than 8 volts.
7. All other AF ECM DTCs for this HO2S sensor have ran and passed.
8. This DTC has not tested this ignition cycle.
9. The AF ECM determines the HO2S was cold at ignition key ON. A cold HO2S is indicated when the average HO2S signal voltage is within the bias voltage range of 0.30 volt (300 mV) to 0.60 volt (600 mV) when the ignition is turned ON.

The AF ECM monitors an HO2S signal voltage that did not leave the cold sensor bias voltage range 0.30 volt (300 mV) to 0.60 volt (600 mV) in an acceptable amount of time. The time varies from 50 seconds to 110 seconds based upon mass air flow.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Connect a scan tool. Set-up the scan tool with the proper vehicle identification information prior to turning the ignition ON. Establish communication with the PCM for this step. Turn the ignition ON, with the engine OFF, and as quickly as possible observe and record the HO2S voltage for the sensor that applies to this DTC. Observe the HO2S voltage for 2 minutes. Does the HO2S voltage increase or decrease from the initial voltage by at least 150 mV? If yes, go to next step. If no, go to step 5 .
3. Allow the HO2S voltage to stabilize. Use the scan tool in order to observe and record the current HO2S voltage from the PCM. Use the scan tool in order to establish communication with the AF ECM. Use the scan tool in order to observe and record the current voltage for the HO2S sensor that applies to this DTC. Does the AF ECM indicate the same HO2S voltage as the PCM, plus or minus 45 mV? If yes, go to next step. If no, go to step 9 .
4. The condition that set this DTC is not present. This DTC may have been set by one of the following conditions: An intermittently open or high resistance HO2S high signal circuit or low circuit. An intermittently open or high resistance HO2S heater ignition feed or ground circuit. A faulty HO2S or PCM connector terminal contact. A defective HO2S. Repair any of the above conditions as necessary. Was a condition found and repaired? If yes, go to step 17 . If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
5. Turn the ignition OFF. Disconnect the HO2S connector for the sensor that applies to this DTC. Connect the Unpowered Test Lamp (J 34142-B) between the HO2S heater ignition feed and ground circuit terminals, engine harness side. Turn the ignition ON, with the engine OFF. Does the test lamp illuminate? If yes, go to next step. If no, go to step 7 .
6. Disconnect the test lamp. Jumper the HO2S low circuit terminal to a known-good ground. Jumper the HO2S high signal circuit terminal to a known-good ground. Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC. Ensure that the ignition is ON while the engine is OFF. Is the voltage less than 20 mV? If yes, go to step 14 . If no, go to step 8 .
7. Connect the test lamp between the HO2S heater ignition feed circuit terminal (engine harness side) and a known-good ground. Ensure that the ignition is ON while the engine is OFF. Does the test lamp illuminate? If yes, go to step 11 . If no, go to step 12 .
8. Turn the ignition OFF. Remove the jumpers from the HO2S terminals. Disconnect the PCM connector containing the HO2S high and low circuits. Use a Digital Multimeter (DMM) in order to measure the resistance of the HO2S high signal circuit and the HO2S low circuit between the PCM connector terminal and the HO2S connector terminal, engine harness side. Are both resistances less than 5 ohms? If yes, go to step 10 . If no, go to step 13 .
9. Turn OFF the ignition. Disconnect the AF ECM connector C002. Disconnect the HO2S sensor connector that applies to this DTC. Test the HO2S high and low circuits for the following conditions: Excessive resistance. Short to ground. Open circuits. Short to other circuits. Did you find and correct the condition? If yes, go to step 17 . If no, go to step 15 .
10. Inspect for proper PCM terminal contact. Repair the terminals as necessary. Was a terminal repair made? If yes, go to step 17 . If no, go to step 16 .
11. Repair the open HO2S heater ground circuit. After repairs, go to step 17 .
12. Repair the open or short to ground on the ignition feed circuit of the HO2S heater. After repairs, go to step 17 .
13. Repair the circuit that measured high resistance. After repairs, go to step 17 .
14. Replace suspect HO2S. See «OXYGEN SENSORS»(ref-150199-S40719737762002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 17 .
15. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to step 17 .
16. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
17. Turn OFF the ignition. Allow the HO2S to cool for 1/2 hour. Connect a scan tool. Set-up the scan tool with the proper vehicle identification information prior to turning the ignition ON. Turn ON the ignition, with the engine OFF, and as quickly as possible observe and record the HO2S voltage for the sensor that applies to this DTC. Observe the HO2S voltage for 2 minutes. Does the voltage increase or decrease from the initial voltage by at least 150 mV? If yes, go to next step. If no, go to step 2 .
18. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The engine coolant temperature is greater than 120°F (49°C).
3. The AF ECM is operating in Closed Loop fuel control.

The AF ECM monitors that the Short Term Fuel Trim is at plus 50 percent or the Long Term Fuel Trim is at plus 45 percent for at least 5 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Idle the engine at the normal operating temperature. The fuel system is in Closed Loop. Observe the Long Term FT Avg. display for bank 1 or bank 2 with a scan tool. Is the Long Term FT Avg. less than 45 percent? If yes, go to next step. If no, go to step 4 .
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Are both banks of the engine operating lean? If yes, go to next step. If no, go to step 6 .
5. Inspect for the following conditions: Inspect the vacuum hoses for splits, kinks, and proper connection. Inspect the crankcase ventilation valve and system for leaks. Inspect the PCM and sensor grounds for being clean, tight, and in the correct locations. Inspect the air intake system after the MAF sensor for vacuum leaks. Inspect for an engine mechanical condition. Did you find and correct the condition? If yes, go to step 7 . If no, see «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
6. Inspect for the following conditions: Inspect for exhaust leaks, missing or loose exhaust hardware. Inspect that the HO2S is installed securely and the electrical connector is not contacting the exhaust system or the ignition wires. Inspect for an engine mechanical condition. Inspect for vacuum leaks that only affect one bank of the engine. For example, the intake manifold, the injector O-rings. Did you find and correct the condition? If yes, go to next step. If no, see «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
7. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
8. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The engine coolant temperature is greater than 120°F (49°C).
3. The AF ECM is operating in Closed Loop fuel control.

The AF ECM monitors that the Short Term Fuel Trim is at -50 percent for at least 5 seconds continuously OR the AF ECM monitors that the LONG Term Fuel Trim is at -45 percent for at least 5 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Idle the engine at the normal operating temperature. The fuel system is in Closed Loop. Observe the Long Term FT Avg. display for bank 1 or bank 2 with a scan tool. Is the Long Term FT Avg. more than -45 percent? If yes, go to next step. If no, go to step 4 .
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Are both banks of the engine operating rich? If yes, go to next step. If no, go to step 6 .
5. Inspect for the following conditions: Inspect for a collapsed air intake duct. Inspect for a restricted air filter element. Inspect the MAF sensor for being installed in the proper direction and for foreign objects blocking the inlet screen. Inspect the exhaust system for a restriction. Did you find and correct the condition? If yes, go to step 7 . If no, see «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
6. Inspect the fuel injectors for leaking. See «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article. Are any fuel injectors leaking? If yes, go to next step. If no, go to step 8 .
7. Repair or replace any faulty items found. After repairs, go to step 9 .
8. Replace suspect HO2S. See «OXYGEN SENSORS»(ref-150199-S40719737762002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
9. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
10. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The Engine Coolant Temperature (ECT) is more than 120°F (49°C).
3. The AF ECM is operating in Closed Loop fuel control.

The AF ECM monitors that the Short Term Fuel Trim is at plus 50 percent or the Long Term Fuel Trim is at plus 45 percent for at least 5 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Idle the engine at the normal operating temperature. The fuel system is in Closed Loop. Observe the Long Term FT Avg. display for bank 1 or bank 2 with a scan tool. Is the Long Term FT Avg. less than 45 percent? If yes, go to next step. If no, go to step 4 .
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Are both banks of the engine operating lean? If yes, go to next step. If no, go to step 6 .
5. Inspect for the following conditions: Inspect the vacuum hoses for splits, kinks, and proper connection. Inspect the crankcase ventilation valve and system for leaks. Inspect the PCM and sensor grounds for being clean, tight, and in the correct locations. Inspect the air intake system after the MAF sensor for vacuum leaks. Inspect for an engine mechanical condition. Did you find and correct the condition? If yes, go to step 7 . If no, see «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
6. Inspect for the following conditions: Inspect for exhaust leaks, missing or loose exhaust hardware. Inspect that the HO2S is installed securely and the electrical connector is not contacting the exhaust system or the ignition wires. Inspect for an engine mechanical condition. Inspect for vacuum leaks that only affect one bank of the engine. For example, the intake manifold, the injector O-rings. Did you find and correct the condition? If yes, go to next step. If no, See «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
7. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
8. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The engine coolant temperature is more than 120°F (49°C).
3. The AF ECM is operating in Closed Loop fuel control.

The AF ECM monitors that the Short Term Fuel Trim is at -50 percent or Long Term Fuel Trim is at -45 percent for at least 5 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Idle the engine at the normal operating temperature. The fuel system is in Closed Loop. Observe the Long Term FT Avg. display for bank 1 or bank 2 with a scan tool. Is the Long Term FT Avg. more than -45 percent? If yes, go to next step. If no, go to step 4 .
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Are both banks of the engine operating rich? If yes, go to next step. If no, go to step 6 .
5. Inspect for the following conditions: Inspect for a collapsed air intake duct. Inspect for a restricted air filter element. Inspect the MAF sensor for being installed in the proper direction and for foreign objects blocking the inlet screen. Inspect the exhaust system for a restriction. Did you find and correct the condition? If yes, go to step 7 . If no, see «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
6. Inspect the fuel injectors for leaking. See «FUEL SYSTEM DIAGNOSIS (BI-FUEL & CNG)»(ref-150429-S03967267952002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article. Are any fuel injectors leaking? If yes, go to next step. If no, go to step 8 .
7. Repair or replace any faulty items found. After repairs, go to step 9 .
8. Replace suspect HO2S. See «OXYGEN SENSORS»(ref-150199-S40719737762002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
9. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
10. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The ignition is ON.

The AF ECM detects that the fuel tank temperature sensor voltage is less than 0.05 V for more than 5 seconds.

The AF ECM records the operating conditions at the time the diagnostic fails. The AF ECM stores this information in the Freeze Frame and/or Failure Records.

1. A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
2. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
3. Use a scan tool in order to clear the MIL and the DTC.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. Observe the FTT parameter with a scan tool. Does the scan tool indicate that the FTT is less than 0.05 V? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «DIAGNOSTIC AIDS»(ref-151060-S00185908112002121100000) .
4. Disconnect the harness connector of the FTT sensor. Turn ON the ignition, with the engine OFF. Observe the FTT parameter with a scan tool. Does the scan tool indicate that the FTT sensor voltage is more than 4.6 V? If yes, go to next step. If no, go to step 7 .
5. Measure the voltage from the 5-volt reference circuit of the FTT sensor with a DMM. Does the DMM display a voltage near 5 V? If yes, go to step 8 . If no, go to next step.
6. Test the 5-volt reference circuit of the FTT sensor for a short to ground. Did you find and correct the condition? If yes, go to step 10 . If no, go to step 9 .
7. Turn OFF the ignition. Disconnect the AF ECM. Test the signal circuit of the FTT sensor for a short to ground. Did you find and correct the condition? If yes, go to step 10 . If no, go to step 9 .
8. Replace the FTT sensor. See HIGH PRESSURE LOCK-OFF SOLENOID under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. After repairs, go to step 10 .
9. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
10. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
11. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

Use the Temperature vs Resistance table in order to test the FTT sensor at various temperature levels in order to evaluate the possibility of a skewed sensor. A skewed sensor could result in a driveability condition. If the engine has sat overnight, the FTT sensor and the intake air temperature values should display within a few degrees. If the temperatures are not within 5°F (3°C), see SENSOR OPERATING RANGE CHARTS - TRUCKS article.

For an intermittent condition, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.

1. The ignition is ON.
2. The intake air temperature is more than 14°F (-10°C).
3. DTCs P0112, and P0113 are not set.

The AF ECM detects that the fuel tank temperature sensor voltage is more than 4.95 V for more than 5 seconds.

The AF ECM records the operating conditions at the time the diagnostic fails. The AF ECM stores this information in the Freeze Frame and/or Failure Records.

1. A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
2. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
3. Use a scan tool in order to clear the MIL and the DTC.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. Observe the FTT sensor parameter with a scan tool. Does the scan tool indicate that the FTT is near 5 V? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «DIAGNOSTIC AIDS»(ref-151060-S06269907922002121100000) .
4. Turn OFF the ignition. Disconnect the FTT sensor. Connect a 3-amp fused jumper wire between the signal circuit of the FTT sensor and the low reference circuit of the FTT sensor. Turn ON the ignition, with the engine OFF. Observe the FTT parameter with a scan tool. Does the scan tool indicate that the FTT is at 0 V? If yes, go to step 8 . If no, go to next step.
5. Turn OFF the ignition. Connect a 3-amp fused jumper wire between the signal circuit of the FTT sensor and a good ground. Turn ON the ignition, with the engine OFF. Observe the FTT parameter with a scan tool. Does the scan tool indicate that the FTT is at 0 V? If yes, go to step 7 . If no, go to next step.
6. Test the signal circuit of the FTT sensor for an open circuit or for high resistance. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 10 .
7. Test the low reference circuit of the FTT sensor for a high resistance or an open. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 10 .
8. Turn OFF the ignition. Connect a 3-amp fused jumper wire between the signal circuit of the FTT sensor and a good ground. Start the engine. Do any additional DTCs set? If yes, go to step 11 . If no, go to next step.
9. Test for an intermittent and for a poor connection at the FTT sensor. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 12 .
10. Test for an intermittent and for a poor connection at the AF ECM. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 13 .
11. Repair the short between the signal circuit of the FTT sensor and the circuit for which the DTC set. After repairs, go to step 14 .
12. Replace the FTT sensor. See HIGH PRESSURE LOCK-OFF SOLENOID under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. After repairs, go to step 14 .
13. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
14. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
15. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

If a short to a separate 5-volt source occurs this DTC may set, if this is found to be the condition a continuity test to all other PCM circuits will be necessary to diagnose the specific circuit.

Use the Temperature vs Resistance table in order to test the FTT sensor at various temperature levels in order to evaluate the possibility of a skewed sensor. A skewed sensor could result in a driveability condition. If the engine has sat overnight, the FTT sensor and the intake air temperature values should display within a few degrees. If the temperatures are not within 5°F (3°C), see SENSOR OPERATING RANGE CHARTS - TRUCKS article.

For an intermittent condition, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.

The ignition is ON.

The AF ECM detects that the fuel rail temperature sensor voltage is less than 0.05 V for more than 5 seconds.

The AF ECM records the operating conditions at the time the diagnostic fails. The AF ECM stores this information in the Freeze Frame and/or Failure Records.

1. A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
2. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
3. Use a scan tool in order to clear the MIL and the DTC.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. Observe the FRT parameter with a scan tool. Does the scan tool indicate that the FRT is less than 0.05 V? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «DIAGNOSTIC AIDS»(ref-151060-S40901778852002121100000) .
4. Disconnect the harness connector of the FRT sensor. Turn ON the ignition, with the engine OFF. Observe the FRT parameter with a scan tool. Does the scan tool indicate that the FRT sensor voltage is more than 4.6 V? If yes, go to step 6 . If no, go to next step.
5. Turn OFF the ignition. Disconnect the PCM. Test the signal circuit of the FRT sensor for a short to ground. Did you find and correct the condition? If yes, go to step 8 . If no, go to next step.
6. Replace the FRT sensor. See FUEL RAIL TEMPERATURE SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. After repairs, go to step 8 .
7. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
8. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
9. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

Use the Temperature vs Resistance table in order to test the FRT sensor at various temperature levels in order to evaluate the possibility of a skewed sensor. A skewed sensor could result in a driveability condition. If the engine has sat overnight, the FRT sensor and the intake air temperature values should display within a few degrees. If the temperatures are not within 5°F (3°C), see SENSOR OPERATING RANGE CHARTS - TRUCKS article.

For an intermittent condition, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.

1. The ignition is ON.
2. The intake air temperature is more than 14°F (-10°C).
3. DTCs P0112, and P0113 are not set.

The AF ECM detects that the fuel rail temperature sensor voltage is more than 4.95 V for more than 5 seconds.

The AF ECM records the operating conditions at the time the diagnostic fails. The AF ECM stores this information in the Freeze Frame and/or Failure Records.

1. A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
2. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
3. Use a scan tool in order to clear the MIL and the DTC.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. Observe the FRT sensor parameter with a scan tool. Does the scan tool indicate that the FRT is at 5 V? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «DIAGNOSTIC AIDS»(ref-151060-S25010089872002121100000) .
4. Turn OFF the ignition. Disconnect the FRT sensor. Connect a 3-amp fused jumper wire between the signal circuit of the FRT sensor and the low reference circuit of the FRT sensor. Turn ON the ignition, with the engine OFF. Observe the FRT parameter with a scan tool. Does the scan tool indicate that the FRT is at 0 V? If yes, go to step 8 . If no, go to next step.
5. Turn OFF the ignition. Connect a 3-amp fused jumper wire between the signal circuit of the FRT sensor and a good ground. Turn ON the ignition, with the engine OFF. Observe the FRT parameter with a scan tool. Does the scan tool indicate that the FRT is at 0 V? If yes, go to step 7 . If no, go to next step.
6. Test the signal circuit of the FRT sensor for an open circuit or for high resistance. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 10 .
7. Test the low reference circuit of the FRT sensor for a high resistance or an open. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 10 .
8. Turn OFF the ignition. Connect a 3-amp fused jumper wire between the signal circuit of the FRT sensor and a good ground. Start the engine. Do any additional DTCs set? If yes, go to step 11 . If no, go to next step.
9. Test for an intermittent and for a poor connection at the FRT sensor. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 12 .
10. Test for an intermittent and for a poor connection at the AF ECM. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 13 .
11. Repair the short between the signal circuit of the FRT sensor and the circuit for which the DTC set. After repairs, go to step 14 .
12. Replace the FRT sensor. See FUEL RAIL TEMPERATURE SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. After repairs, go to step 14 .
13. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
14. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
15. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

If a short to a separate 5-volt source occurs this DTC may set, if this is found to be the condition a continuity test to all other PCM circuits will be necessary to diagnose the specific circuit.

Use the Temperature vs Resistance table in order to test the FRT sensor at various temperature levels in order to evaluate the possibility of a skewed sensor. A skewed sensor could result in a driveability condition. If the engine has sat overnight, the FRT sensor and the intake air temperature values should display within a few degrees. If the temperatures are not within 5°F (3°C), see SENSOR OPERATING RANGE CHARTS - TRUCKS article.

For an intermittent condition, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.

1. The engine speed is greater than 100 RPM.
2. AF ECM DTC P0336 is not set.

The AF ECM monitors a FRP sensor signal voltage of less than 0.05 volt for at least 10 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The F ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. With a scan tool establish communication with the AF ECM. Observe the Fuel Rail Pressure (FRP) data display. Does the scan tool display the FRP sensor voltage less than 0.5 V? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Disconnect the FRP sensor. Observe the FRP sensor data display with a scan tool. Does the scan tool display the FRP sensor voltage more than 4.95 V? If yes, go to next step. If no, go to step 6 .
5. Measure the voltage from the 5-volt reference circuit of the FRP sensor with a DMM. Is the voltage less than 4.95 V? If yes, go to step 7 . If no, go to step 8 .
6. Test for a short to ground on the signal circuit of the FRP sensor. Did you find and correct the condition? If yes, go to step 12 . If no, go to step 9 .
7. Test for an open or a short to ground on the 5-volt reference circuit of the FRP sensor. Disconnect the Fuel Tank Pressure (FTP) sensor in order to determine if the FTP sensor is the cause of a short. Did you find and correct the condition? If yes, go to step 12 . If no, go to step 9 .
8. Inspect for proper terminal contact at the harness connector of the FRP sensor. Did you find and correct the condition? If yes, go to step 12 . If no, go to step 10 .
9. Inspect for proper terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 12 . If no, go to step 11 .
10. Replace the fuel rail pressure sensor. See FUEL RAIL PRESSURE SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. After repairs, go to step 12 .
11. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
12. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
13. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine speed is greater than 100 RPM.
2. AF ECM DTC P0336 is not set.

The AF ECM monitors a FRP sensor signal voltage of more than 4.95 volts for at least 10 seconds continuously.

1. A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. With a scan tool establish communication with the AF ECM. Observe the Fuel Rail Pressure (FRP) data display. Does the scan tool display the FRP sensor voltage more than 4.95 V? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Observe the Fuel Rail Temperature (FRT) sensor data display with a scan tool. Does the scan tool display the FRT sensor voltage more than 4.7 V? If yes, go to step 8 . If no, go to next step.
5. Disconnect the FRP sensor. Connect a 3-amp fused jumper wire between the signal circuit of the FRP sensor and a known-good ground. Observe the FRP sensor data display with a scan tool. Does the scan tool display the FRP sensor voltage less than 0.05 V? If yes, go to next step. If no, go to step 9 .
6. Remove the jumper. Measure the voltage from the 5-volt reference circuit of the FRP sensor with a DMM. Does the DMM display a voltage more than 5.2 V? If yes, go to next step. If no, go to step 10 .
7. Probe the ground circuit of the FRP sensor with a test lamp connected to battery voltage. Does the test lamp illuminate? If yes, go to step 11 . If no, go to step 13 .
8. Test the ground circuit of the FRP sensor for an open between the AF ECM and the splice. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 12 .
9. Test the signal circuit of the FRP sensor for a short to voltage or an open. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 12 .
10. Test the 5-volt reference circuit of the FRP sensor for a short to voltage. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 15 .
11. Inspect for proper terminal contact at the harness connector of the FRP sensor. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 14 .
12. Inspect for proper terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 15 .
13. Repair the ground circuit of the FRP sensor for an open, or high resistance, between the harness connector of the FRP sensor and the splice. After repairs, go to step 16 .
14. Replace the fuel rail pressure sensor. See FUEL RAIL PRESSURE SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. After repairs, go to step 16 .
15. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
16. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
17. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The engine is operating on CNG.

1. The AF ECM detects an incorrect voltage on a fuel injector control circuit.
2. The condition exists for 5 seconds.

1. The AF ECM illuminates the Malfunction Indicator Lamp (MIL) when the diagnostic runs and fails.
2. The AF ECM records the operating conditions at the time the diagnostic fails. The AF ECM stores this information in the Freeze Frame and/or Failure Records.
3. The AF ECM will switch over to gasoline operation (bi-fuel).

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. With a scan tool clear the DTCs. Idle the engine at the normal operating temperature. With a scan tool observe the alternative fuels Diagnostic Trouble Code (DTC) information. Does the scan tool indicate that any of the DTCs P0201-P0208 set this ignition? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «DIAGNOSTIC AIDS»(ref-151060-S38078330462002121100000) .
4. Does the condition affect an entire bank of the engine? If yes, go to step 10 . If no, go to next step.
5. Turn OFF the ignition. Disconnect the appropriate harness connector of the fuel injector. Turn ON the ignition, with the engine OFF. Probe the ignition voltage circuit of the fuel injector with a test lamp connected to a good ground. Does the test lamp illuminate? If yes, go to next step. If no, go to step 17 .
6. Connect the Fuel Injector Test Lamp (J 34730-2C) between the control circuit and the ignition voltage circuit of the fuel injector harness connector. Start the engine. Does the test lamp blink? If yes, go to step 15 . If no, go to next step.
7. Does the test lamp remain illuminated at all times? If yes, go to step 9 . If no, go to next step.
8. Test the control circuit of the fuel injector for a short to voltage or an open. Did you find and correct the condition? If yes, go to step 23 . If no, go to step 16 .
9. Test the control circuit of the fuel injector for a short to ground. Did you find and correct the condition? If yes, go to step 23 . If no, go to step 22 .
10. Turn OFF the ignition. Disconnect the appropriate fuel injector relay. Turn ON the ignition, with the engine OFF. Probe the ignition voltage circuit of the injector relay coil with a test lamp connected to a good ground. Does the test lamp illuminate? If yes, go to next step. If no, go to step 18 .
11. Probe the battery voltage circuit of the injector relay switch with a test lamp connected to a good ground. Does the test lamp illuminate? If yes, go to next step. If no, go to step 18 .
12. Probe the ground circuit of the injector relay with a test lamp connected to battery voltage. Does the test lamp illuminate? If yes, go to next step. If no, go to step 19 .
13. Test the ignition feed circuit of the fuel injectors for an open between the fuel injector relay and the splice. Did you find and correct the condition? If yes, go to step 23 . If no, go to next step.
14. Inspect for poor connections at the harness connector of the fuel injector relay. Did you find and correct the condition? If yes, go to step 23 . If no, go to step 20 .
15. Inspect for poor connections at the harness connector of the fuel injector. Did you find and correct the condition? If yes, go to step 23 . If no, go to step 21 .
16. Inspect for poor connections at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 23 . If no, go to step 22 .
17. Repair the ignition voltage circuit of the fuel injector. After repairs, go to step 23 .
18. Repair the voltage circuit of the fuel injector relay. After repairs, go to step 23 .
19. Repair the ground circuit of the fuel injector relay. After repairs, go to step 23 .
20. Replace the appropriate fuel injector relay. After repairs, go to step 23 .
21. Replace the fuel injector. See FUEL INJECTOR under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. After repairs, go to step 23 .
22. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
23. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
24. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

Performing the Fuel Injector Coil test may help isolate an intermittent condition. See FUEL INJECTOR COIL TEST (BI-FUEL & CNG) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.

For an intermittent condition, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.

1. No AF ECM MAF sensor DTCs are set this ignition cycle.
2. The MAP sensor signal indicates 31.17 in. Hg (105.23 kPa) or less.
3. The mass air flow is more than 3 grams per second for 1 second continuously.

The AF ECM does not monitor an engine speed signal during the 1 second that mass air flow is more than 3 grams per second.

1. The AF ECM will switchover to gasoline operation on bi-fuel equipped vehicles.
2. The AF ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Crank and attempt to start the engine. Does the engine start and continue to run? If yes, go to next step. If no, go to step 5 .
3. With a scan tool, establish communication with the PCM, and review DTC information. Is DTC P0335 stored in the PCM? If yes, see «DTC P0335: CRANKSHAFT POSITION SENSOR CIRCUIT»(ref-138163-S04834272012002032500000) under DIAGNOSTIC TESTS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C" & "K" SERIES TRUCKS. If no, go to next step.
4. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to step 12 . If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
5. With a scan tool, establish communications with the PCM. Observe the RPM data display. Crank the engine. Does the scan tool display RPM as the engine is cranking? If yes, go to step 12 . If no, go to next step.
6. Turn OFF the ignition. Disconnect the CKP sensor. Turn ON the ignition, with the engine OFF. Measure voltage from the ignition feed circuit of the CKP sensor with a DMM. Does the DMM display battery voltage? If yes, go to next step. If no, go to step 13 .
7. Measure the voltage between the ignition feed circuit and the low reference circuit of the CKP sensor with a DMM. Does the DMM display battery voltage? If yes, go to next step. If no, go to step 14 .
8. Observe the RPM data display, in the PCM data list, with a scan tool. Momentarily probe the sensor circuit of the CKP sensor with a test lamp connected to battery voltage. Does the RPM data display increment when the test lamp contacts the sensor circuit? If yes, go to step 15 . If no, go to next step.
9. Turn OFF the ignition. Disconnect the PCM connector C1. Turn ON the ignition, with the engine OFF. Measure the voltage from the CKP sensor signal circuit with a DMM. Does the DMM display near 1.5 Volts? If yes, go to step 20 . If no, go to next step.
10. Test the CKP sensor signal circuit for a short to ground. Did you find and correct the condition? If yes, go to step 21 . If no, go to next step.
11. Test the CKP sensor signal circuit for an open or a short to voltage. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 16 .
12. Test the CKP sensor signal circuit for an open between the AF Fuels ECM and the splice. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 16 .
13. Test the ignition feed circuit of the CKP sensor for an open. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 17 .
14. Test the low reference circuit of the CKP sensor for an open. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 17 .
15. Inspect for proper terminal contact at the harness connector of the CKP sensor. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 18 .
16. Inspect for proper terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 19 .
17. Inspect for proper terminal contact at the harness connector of the PCM. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 20 .
18. Replace the CKP sensor. See «CRANKSHAFT POSITION SENSOR»(ref-150199-S00479426712002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 21 .
19. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to step 21 .
20. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
21. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
22. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. DTCs P0101, P0102, P0103 are not set.
2. MAP value is less than 105 kPa.
3. The MAF is greater than 3 gms/sec.

1. The AF ECM does not detect a CMP sensor reference pulse.
2. The condition exists for at least 2 seconds.

1. The control module illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
2. The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

1. The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
2. A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
3. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
4. Clear the MIL and the DTC with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. With a scan tool, establish communication with the AF ECM. Start and idle the engine for 2 minutes. Monitor the AF ECM DTC information with a scan tool. Does the scan tool indicate that DTC P0341 failed this ignition? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. With a scan tool, establish communication with the PCM. Monitor the CMP high to low and the low to high transition parameters with the scan tool. Do the scan tool parameters increment? If yes, go to step 10 . If no, go to next step.
5. Turn OFF the ignition. Disconnect the CMP sensor. Turn ON the ignition, with the engine OFF. Probe the CMP ignition feed circuit with a test lamp connected to a good ground. Does the test lamp illuminate? If yes, go to next step. If no, go to step 11 .
6. Probe the CMP sensor low reference circuit with a test lamp connected to battery voltage. Does the test lamp illuminate? If yes, go to next step. If no, go to step 12 .
7. With a scan tool, establish communication with the PCM. Monitor the CMP high to low and the low to high transition parameters with the scan tool. With a test lamp connected to battery voltage, momentarily probe the CMP sensor signal circuit. Do the fuel injectors pulse when the test lamp contacts the signal circuit? If yes, go to step 14 . If no, go to next step.
8. Turn OFF the ignition. Disconnect the AF ECM connector C001. Turn ON the ignition, with the engine OFF. Monitor the CMP high to low and the low to high transition parameters with the scan tool. With a test lamp connected to battery voltage, momentarily and repeatedly probe the CMP sensor signal circuit. Do the scan tool parameters increment when the test lamp contacts the signal circuit? If yes, go to step 17 . If no, go to next step.
9. Test the CMP sensor signal circuit for an open, short to ground, or a short to voltage. Did you find and correct the condition? If yes, go to step 19 . If no, go to step 15 .
10. Test the CMP sensor signal circuit for an open between the AF ECM and the splice. Did you find and correct the condition? If yes, go to step 19 . If no, go to step 13 .
11. Test the ignition feed circuit of the CMP sensor for an open or a short to ground. Did you find and correct the condition? If yes, go to step 19 . If no, go to step 15 .
12. Test the ground circuit of the CMP sensor for an open. Did you find and correct the condition? If yes, go to step 19 . If no, go to step 15 .
13. Inspect for proper terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 19 . If no, go to step 17 .
14. Inspect for proper terminal contact at the harness connector of the CMP. Did you find and correct the condition? If yes, go to step 19 . If no, go to step 16 .
15. Inspect for proper terminal contact at the harness connector of the PCM. Did you find and correct the condition? If yes, go to step 19 . If no, go to step 18 .
16. Replace the CMP sensor. See «CAMSHAFT POSITION SENSOR»(ref-150199-S34835402952002110500000) under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 19 .
17. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to step 19 .
18. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
19. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
20. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The vehicle is operating on CNG.

1. The EGR valve position signal voltage is less than 0.09 volt.
2. The condition exists for more than 5 seconds.

1. The control module illuminates the Malfunction Indicator Lamp (MIL) when the diagnostic runs and fails.
2. The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

1. The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
2. A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
3. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
4. Clear the MIL and the DTC with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. With a scan tool, establish communication with the PCM. Observe the Diagnostic Trouble Code (DTC) information. Does the scan tool display DTC P1635? If yes, see «DTC P1635: 5-VOLT REFERENCE 1 CIRCUIT»(ref-151060-S11736825112002121200000) . If no, go to next step.
3. With a scan tool, establish communication with the AF ECM. Observe the EGR position sensor voltage parameter. Does the scan tool indicate that the EGR position sensor voltage is less than 0.09 V? If yes, go to step 5 . If no, go to next step.
4. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
5. Turn OFF the ignition. Disconnect the harness connector of the EGR valve. Turn ON the ignition, with the engine OFF. Measure the voltage from the 5-volt reference circuit of the EGR position sensor with a DMM. Does the DMM display a voltage near 5 V? If yes, go to next step. If no, go to step 11 .
6. Turn OFF the ignition. Connect a fused jumper wire between the 5-volt reference circuit and the signal circuit of the EGR position sensor. Turn ON the ignition, with the engine OFF. With a scan tool, observe the EGR position sensor voltage parameter from the AF ECM and the PCM. Does the scan tool indicate that the EGR position sensor voltage is at 0 V in both data lists? If yes, go to step 9 . If no, go to next step.
7. Does the scan tool indicate that the EGR position sensor voltage is at 5 V in both data lists? If yes, go to step 13 . If no, go to next step.
8. Test the signal circuit of the EGR for an open between the AF ECM and the splice. Did you find and correct the condition? If yes, go to step 18 . If no, go to step 14 .
9. Turn OFF the ignition. Disconnect the AF ECM connector C002. Turn ON the ignition, with the engine OFF. With a scan tool, establish communication with the PCM. Observe the EGR position sensor voltage parameter. Does the scan tool indicate that the EGR position sensor voltage is near 5 V? If yes, go to step 17 . If no, go to next step.
10. Test the signal circuit of the EGR pintle position for a short to ground, or a high resistance. Did you find and correct the condition? If yes, go to step 18 . If no, go to step 16 .
11. Test the 5-volt reference circuit of the EGR position sensor for an open or high resistance. Did you find and correct the condition? If yes, go to step 18 . If no, go to next step.
12. Inspect for proper connections at the harness connector of the PCM. Did you find and correct the condition? If yes, go to step 18 . If no, go to step 16 .
13. Inspect for proper connections at the harness connector of the EGR valve. Did you find and correct the condition? If yes, go to step 18 . If no, go to step 15 .
14. Inspect for proper connections at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 18 . If no, go to step 17 .
15. Replace the EGR valve. See «EGR VALVE»(ref-150199-S09843844812002111100000) under EMISSION SYSTEMS & SUB-SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 18 .
16. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 18 .
17. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
18. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
19. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The vehicle is operating on CNG.

1. The EGR valve position signal voltage is more than 4.9 volts.
2. The condition exists for more than 5 seconds.

1. The control module illuminates the Malfunction Indicator Lamp (MIL) when the diagnostic runs and fails.
2. The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

1. The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
2. A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
3. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
4. Clear the MIL and the DTC with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. With a scan tool, establish communication with the AF ECM. Observe the EGR position sensor voltage parameter. Does the scan tool indicate that the EGR position sensor voltage is more than 4.9 V? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Turn OFF the ignition. Disconnect the harness connector of the EGR valve. Turn ON the ignition, with the engine OFF. Observe the EGR position sensor voltage parameter with a scan tool. Does the scan tool indicate that the EGR position sensor voltage is more than 4.9 V? If yes, go to step 8 . If no, go to next step.
5. Measure the voltage from the 5-volt reference circuit of the EGR position sensor with a DMM. Does the DMM display a voltage more than 5.2 V? If yes, go to step 10 . If no, go to next step.
6. Probe the low reference circuit of the EGR pintle position sensor with a test lamp connected to battery voltage. Does the test lamp illuminate? If yes, go to step 12 . If no, go to next step.
7. Test the low reference of the EGR pintle position sensor for an open. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 11 .
8. Turn OFF the ignition. Disconnect the PCM connector C1. Turn ON the ignition, with the engine OFF. Observe the EGR position sensor voltage parameter with a scan tool. Does the scan tool indicate that the EGR position sensor voltage is more than 4.9 V? If yes, go to next step. If yes, go to step 13 .
9. Test the signal circuit of the EGR pintle position for a short to voltage. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 14 .
10. Test the 5-volt reference circuit of the EGR position sensor for a short to voltage. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 13 .
11. Inspect for proper connections at the harness connector of the PCM. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 13 .
12. Replace the EGR valve. See «EGR VALVE»(ref-150199-S09843844812002111100000) under EMISSION SYSTEMS & SUB-SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 15 .
13. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 15 .
14. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
15. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
16. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.

The AF ECM monitors a vehicle speed output signal circuit fault.

1. A first failure of this DTC will not illuminate the MIL and will store as Last Test Failed but not as History.
2. The AF ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Using the scan tool, establish communication with the PCM. Inspect for DTC P0608. Is DTC P0608 stored as History? If yes, diagnose this DTC first. See appropriate ANALOG INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Test the vehicle speed output circuit for an open condition between the circuit splice and the AF ECM connector. Inspect the AF ECM connector for poor terminal contact. Repair circuit or terminal contact as necessary. Was a circuit or terminal repair made? If yes, go to step 7 . If no, go to next step.
5. Inspect for poor terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 7 . If no, go to next step.
6. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
7. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
8. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The ignition is ON.

1. The AF ECM does not receive, or send, a Class 2 message.
2. The condition exists for 10 seconds.

The AF ECM records the operating conditions at the time the diagnostic fails. The AF ECM stores this information in the Freeze Frame and/or Failure Records.

1. A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
2. A history DTC will clear after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
3. Use a scan tool in order to clear the DTC.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. With a scan tool, attempt to communicate with the AF ECM. Does the scan tool display AF ECM data? If yes, go to next step. If no, go to step 4 .
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Attempt to establish communication with the listed control modules. If you are using a Tech 2, obtain the information with the Class 2 Message Monitor feature: The Instrument Panel Cluster (IPC). The Powertrain Control Module (PCM). The Vehicle Theft Deterrent (VTD). Does the scan tool communicate with all of the listed control modules? If yes, go to next step. If no, go to step 6 .
5. Attempt to start the engine. Does the engine start and idle? If yes, go to step 7 . If no, go to step 9 .
6. Turn OFF the ignition. Disconnect the AF ECM connector C001. Turn ON the ignition, with the engine OFF. Attempt to establish communication with the listed control modules. IPC. PCM. VTD. Does the scan tool communicate with all the listed control modules? If yes, go to step 15 . If no, go to step 8 .
7. Test the class 2 serial data circuit for the following: An open circuit between the AF Fuels ECM and the splice pack SP205. SP205 is located on left side of instrument panel behind headlight switch. Proper connection at the AF ECM or the splice pack SP205. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 15 .
8. Test the class 2 serial data circuit for a short to ground between the AF ECM and the splice pack SP205. Did you find and correct the condition? If yes, go to step 16 . If no, see SCAN TOOL DOES NOT COMMUNICATE WITH CLASS 2 DEVICE in appropriate BODY CONTROL MODULES article in ACCESSORIES & EQUIPMENT.
9. Inspect the following fuses. IGN 0. IGN E. ECM B. Are any of the listed fuses open? If yes, go to next step. If no, go to step 11 .
10. Test the affected circuit for a short to ground. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 15 .
11. Turn OFF the ignition. Disconnect the AF ECM. Turn ON the ignition, with the engine OFF. Probe each voltage supply circuit of the AF ECM with a test lamp connected to a known-good ground. Does the test lamp illuminate for each circuit? If yes, go to next step. If no, go to step 14 .
12. Probe each ground circuit of the AF ECM with a test lamp connected to battery voltage. Does the test lamp illuminate for each circuit? If yes, go to next step. If no, go to step 14 .
13. Inspect for proper connections at the harness connectors of the AF ECM. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 15 .
14. Repair the circuit that did not illuminate the test lamp. After repairs, go to step 16 .
15. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
16. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
17. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Is DTC P0602 set? If yes, go to next step. If no, go to step 5 .
3. Program the AF ECM. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. Does DTC P0602 reset? If yes, go to next step. If no, go to step 7 .
4. Verify that all tool connections are secure. Verify the programming equipment is operating correctly. Verify the correct software/calibration package is used. Attempt to program the AF ECM. Does DTC P0602 reset? If yes, go to next step. If no, go to step 7 .
5. Test for proper power and ground supply to the AF ECM. Inspect for proper terminal contact at the AF ECM connector. Did you find and correct the condition? If yes, go to step 7 . If no, go to next step.
6. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
7. Clear the DTCs with a scan tool. Turn OFF the ignition for 30 seconds. Start the engine. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
8. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The ignition is ON.

1. The transmission range select lever switch indicates an illegal position.
2. The AF ECM detects an illegal range for 30 seconds.

1. The AF ECM illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
2. The AF ECM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the AF ECM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the AF ECM records the operating conditions at the time of the failure. The AF ECM writes the conditions to the Freeze Frame and updates the Failure Records.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Apply the parking brake. Turn ON the ignition, with the engine OFF. With a scan tool, establish communication with the AF ECM. Observe the TR Sw. display while selecting each transmission range: P, R, N, D, 3, 2, and 1. Does the TR Sw. parameter display an error in any range? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «DIAGNOSTIC AIDS»(ref-151060-S40191786172002121300000) .
4. With a scan tool, observe the Transmission Data Display. Observe the TR Sw. display while selecting each transmission range: "P", "R", "N", "D", "3", "2", and "1". Does each selected transmission range match the scan tool TR Sw. display? If yes, go to step 9 . If no, go to next step.
5. With the scan tool, observe the TR Sw. A/B/C/P display. Does the TR Sw. A/B/C/P parameter indicate HI for all range signal states? If yes, go to step 14 . If no, go to next step.
6. Turn OFF the ignition. Disconnect the TR switch connector C1. Turn ON the ignition, with the engine OFF. Does the TR Sw. A/B/C/P parameter indicate HI for all range signal states? If yes, go to next step. If no, go to step 10 .
7. Measure the voltage from each signal circuit of the TR switch with a DMM. Does the DMM display near 10.5 V for each circuit? If yes, go to next step. If no, go to step 12 .
8. Jumper each signal circuit of the TR switch, one at a time, to a known-good ground. With the scan tool, observe the TR Sw. A/B/C/P display as each circuit is grounded. Do more than one circuit display LOW when a circuit is grounded? If yes, go to step 13 . If no, go to step 15 .
9. Turn OFF the ignition. Disconnect the AF ECM connector C001. Disconnect the TR switch connector C1. Turn ON the ignition, with the engine OFF. Measure the voltage from each signal circuit of the TR switch, at the harness connector of the AF ECM, with a DMM. Does the DMM display near 10.5 V for each circuit? If yes, go to step 17 . If no, go to step 18 .
10. Turn OFF the ignition. Disconnect the AF ECM connector C001. With the scan tool, observe the TR Sw. A/B/C/P display. Does the TR Sw. A/B/C/P parameter indicate HI for all range signal states? If yes, go to step 21 . If no, go to next step.
11. Test the signal circuit of the TR switch that did not indicate HI for a short to ground. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 20 .
12. Test the signal circuit of the TR switch that did not display a voltage for an open. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 16 .
13. Test the affected signal circuits of the TR switch for a shorted together condition. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 20 .
14. Test the ground circuit of the TR switch for an open. Did you find and correct the condition? If yes, go to step 22 . If no, go to next step.
15. Inspect for proper connections at the harness connector of the TR switch. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 19 .
16. Inspect for proper connections at the harness connector of the PCM. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 20 .
17. Inspect for proper connections at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 22 . If no, go to step 21 .
18. Repair the signal circuit of the TR switch, that did not illuminate the test lamp, for an open between the AF ECM and the splice. After repairs, go to step 22 .
19. Replace the TR switch. See «PARK/NEUTRAL POSITION SWITCH»(ref-138559-S31260039142002041700000) under REMOVAL & INSTALLATION under AUTOMATIC - "C" & "K" SERIES (EXCEPT ALLISON) in TRANSMISSION SERVICING article. After repairs, go to step 22 .
20. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to step 22 .
21. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
22. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
23. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

This DTC will set if the gear shift cable, or PNP switch is not adjusted properly. Also, if the gear shift lever is held between gears, this DTC will set. See PARK/NEUTRAL POSITION SWITCH and SHIFT CABLE under ADJUSTMENTS in AUTOMATIC - "C" & "K" SERIES (EXCEPT ALLISON).

The table below is the normal state that each gear range should indicate. LOW indicates the circuit is low, switched or shorted to a ground. HI indicates the circuit is high or open. Another parameter to display is the TR Sw. A/B/C/P range. This parameter displays the current gear range the transmission is in. When there is an invalid gear range, the parameter indicates Invalid.

TRANSMISSION RANGE SWITCH VALID INPUT COMBINATIONS

The engine is cranking or running.

The AF ECM monitors a control circuit fault for at least 2 second continuously.

1. The AF ECM will switch over to gasoline operation on bi-fuel equipped vehicles.
2. The AF ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
4. All DTC information can be cleared with a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. With the scan tool Alt. Fuel Engine Data selection, observe the AFO Signal and the AFO Signal Command displays. Do both displays indicate the same information? If yes, go to next step. If no, go to step 6 .
3. Turn the ignition OFF for 30 seconds. Listen for audible operation of the fuel tank High Pressure Lock-Off (HPL) solenoid as you rapidly turn the ignition back and forth between LOCK and RUN. Does the HPL solenoid operate? If yes, go to next step. If no, go to step 8 .
4. With a scan tool clear the Alt. Fuels DTC information. Feel or listen for audible operation of the underhood Low Pressure Lock-Off (LPL) by performing the following: On bi-fuel models, start and idle the engine and with the scan tool Alt. Fuel Special Function selection command the LPL solenoid ON and OFF. On CNG only models, feel or listen for audible operation of the underhood LPL as you crank the engine. The engine may not start. Turn the ignition OFF for 30 seconds prior to every attempt to energize the LPL solenoid by cranking. On all models, does the LPL solenoid operate as the ignition is cranked or commanded? If yes, go to next step. If no, go to step 12 .
5. Start and idle the engine. With the scan tool Alt. Fuel Special Function selection, command the HPL solenoid ON and OFF. Listen for audible operation of the fuel tank HPL solenoid as you command the HPL solenoid ON and OFF. Does the HPL solenoid operate as commanded? If yes, see «DIAGNOSTIC AIDS»(ref-151060-S02061184212002120600000) . If no, go to step 15 .
6. Turn the ignition OFF. Disconnect the AF ECM connectors. Turn ON the ignition with the engine OFF. With the scan tool PCM Engine Data selection, observe the AFO signal display. Does the display indicate PCM? If yes, go to next step. If no, go to step 16 .
7. With a 3-amp fused jumper wire, ground the AFO signal circuit at the AF ECM connector terminal. If the jumper fuse opens repair the short to voltage on the AFO signal circuit. With the scan tool PCM Engine Data selection, observe the AFO signal display. Does the display indicate AFECM? If yes, go to step 23 . If no, go to step 17 .
8. With the Unpowered Test Lamp (J 34142-B) test the AFS Maxifuse(R) in the underhood fuse block. Is the fuse okay? If yes, go to next step. If no, see «FUEL LOCK-OFF RELAY»(ref-150429-S07542980042002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
9. With the scan tool PCM Special Function selection, command the fuel pump ON and OFF. Listen for audible operation of the Lock-off Solenoid Relay. Does the relay operate ON and OFF as commanded? If yes, go to next step. If no, see «FUEL LOCK-OFF RELAY»(ref-150429-S07542980042002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
10. Turn OFF the ignition. Remove the tank cover. Disconnect the HPL solenoid connector. Probe the HPL power circuit terminal with the Unpowered Test Lamp (J 34142-B) connected to a known-good ground. Observe the test lamp as you turn ON the ignition with the engine OFF. Did the test lamp illuminate for about 2 seconds and then turn OFF? If yes, go to next step. If no, go to step 18 .
11. Turn OFF the ignition. Probe the HPL power circuit terminal with the Unpowered Test Lamp (J 34142-B) connected to the HPL solenoid control circuit. Observe the test lamp as you turn ON the ignition with the engine OFF. Did the test lamp illuminate for about 2 seconds and then turn OFF? If yes, go to step 25 . If no, go to step 19 .
12. Turn OFF the ignition. Disconnect the LPL solenoid connector. Probe the LPL power circuit terminal with the Unpowered Test Lamp (J 34142-B) connected to a known-good ground. Observe the test lamp as you turn ON the ignition with the engine OFF. Did the test lamp illuminate for about 2 seconds and then turn OFF? If yes, go to next step. If no, go to step 20 .
13. Turn OFF the ignition. Probe the LPL solenoid control circuit terminal with the Unpowered Test Lamp (J 34142-B) connected to battery voltage. Turn ON the ignition with the engine OFF. Observe the test lamp. Is the test lamp illuminated? If yes, go to step 21 . If no, go to next step.
14. Observe the test lamp as you crank the engine. Does the test lamp illuminate while the engine is cranking? If yes, go to step 26 . If no, go to step 22 .
15. Test the HPL Solenoid Control circuit for a short to ground. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 29 .
16. Test the AFO signal circuit for a short to ground. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 30 .
17. Inspect the PCM for the correct AF software. Program the PCM if necessary. Test the AFO Signal circuit for continuity. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 24 .
18. Test the HPL solenoid power circuit for continuity between the solenoid and the fuel lock-off relay. Did you find and correct the condition? If yes, go to step 31 . If no, see «FUEL LOCK-OFF RELAY»(ref-150429-S07542980042002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
19. Test the HPL solenoid control circuit for continuity or a short to ground between the solenoid and the AF ECM. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 23 .
20. Test the LPL solenoid power circuit for a continuity between the solenoid and the fuel lock-off relay. Did you find and correct the condition? If yes, go to step 31 . If no, see «FUEL LOCK-OFF RELAY»(ref-150429-S07542980042002111300000) under FUEL SYSTEM (BI-FUEL & CNG) in SYSTEM & COMPONENT TESTING - TRUCKS - EXCEPT TRACKER article.
21. Test the LPL solenoid control circuit for a short to ground. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 29 .
22. Test the LPL solenoid control circuit for continuity between the solenoid and the AF ECM. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 23 .
23. Inspect for proper terminal contact at the AF ECM connector. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 29 .
24. Inspect for proper terminal contact at the PCM connector. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 30 .
25. Inspect for proper terminal contact at the HPL connector. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 27 .
26. Inspect for proper terminal contact at the LPL connector. Did you find and correct the condition? If yes, go to step 31 . If no, go to step 28 .
27. Replace the HPL solenoid. After repairs, go to step 31 .
28. Replace the LPL solenoid. After repairs, go to step 31 .
29. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to step 31 .
30. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
31. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
32. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

This DTC may also set after service programming of the Powertrain Control Module (PCM). Clear DTC information after PCM programming is complete.

The scan tool Output Driver data displays are only functional when the vehicle is operating on CNG.

For an intermittent condition, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.

The engine is operating on alternative fuel.

The AF ECM monitors an FTP signal voltage of less than 0.5 volt for at least 1 second continuously.

1. AF ECM will not illuminate the MIL.
2. AF ECM will store the DTC as History during ONE drive trip in which the diagnostic runs and fails.
3. AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The History status will clear after 40 consecutive warm-up cycles with no failures of any DTC.
3. All DTC information can be cleared using a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ignition on. Using scan tool, communicate with AF ECM. Observe fuel tank pressure sensor voltage. Does the fuel tank pressure sensor indicate voltage less than 0.5 volt? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Remove fuel tank cover. Disconnect the FTP sensor connector. Observe fuel tank pressure sensor data parameter. Does the fuel tank pressure sensor indicate voltage greater than 4.7 volts? If yes, go to next step. If no, go to step 6 .
5. Measure the voltage from the 5-volt reference circuit of the FTP sensor with a DMM. Is the voltage less than 4.8 volts? If yes, go to step 7 . If no, go to step 8 .
6. Test for a short to ground on the signal circuit of the FTP sensor. Did you find and correct the condition? If yes, go to step 12 . If no, go to step 9 .
7. Test for an open or a short to ground on the 5-volt reference circuit of the FTP sensor. Did you find and correct the condition? If yes, go to step 12 . If no, go to step 9 .
8. Inspect for proper terminal contact at the harness connector of the FTP sensor. Did you find and correct the condition? If yes, go to step 12 . If no, go to step 10 .
9. Inspect for proper terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 12 . If no, go to step 11 .
10. Replace FTP sensor. See FUEL TANK PRESSURE SENSOR under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. After repairs, go to step 12 .
11. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
12. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
13. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

1. The engine is operating on alternative fuel.
2. The fuel temperature is less than 86°F (30°C).

The AF ECM monitors an FTP sensor signal voltage of greater than 4.70 volts for at least 1 second continuously.

1. AF ECM will not illuminate the MIL.
2. AF ECM will store the DTC as History during ONE drive trip in which the diagnostic runs and fails.
3. AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The History status will clear after 40 consecutive warm-up cycles with no failures of any DTC.
3. All DTC information can be cleared using a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ignition on, with engine off. Using scan tool, establish communication with the AF ECM. Locate and observe the Fuel Tank Pressure (FTP) sensor data parameter. Does the FTP sensor indicate voltage greater than 4.7 volts? If yes, go to step 4 . If no, go to next step.
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Locate and observe the Fuel Tank Temperature (FTT) sensor data parameter. Does the FTT sensor indicate voltage greater than 4.7 volts? If yes, go to step 8 . If no, go to next step.
5. Remove the fuel tank cover. Disconnect the FTP sensor connector. Connect a fused jumper between the FTP sensor signal circuit terminal and a known-good ground. If the jumper fuse opens, repair the signal circuit shorted to battery voltage. Observe the FTP sensor data parameter. Does the FTP sensor indicate voltage less than 0.25 volt? If yes, go to next step. If no, go to step 9 .
6. Remove the jumper. Measure the voltage from the 5-volt reference circuit of the FTP sensor with a DMM. Is voltage greater than 5.2 volts? If yes, go to step 10 . If no, go to next step.
7. Probe the ground circuit of the FTP sensor with a test lamp connected to battery voltage. Does the test lamp illuminate? If yes, go to step 11 . If no, go to step 13 .
8. Test the ground circuit of the FTP sensor for an open between the AF ECM and the splice. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 12 .
9. Test the signal circuit of the FTP sensor for a short to voltage or an open. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 12 .
10. Test the FTP sensor 5-volt reference circuit for a short to voltage. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 15 .
11. Inspect the FTP sensor connector for proper terminal contact. Repair terminal contact as necessary. Was a repair necessary? If yes, go to step 16 . If no, go to step 14 .
12. Inspect for proper terminal contact at the harness connector of the AF ECM. Did you find and correct the condition? If yes, go to step 16 . If no, go to step 15 .
13. Repair the open or high resistance condition of the sensor ground circuit between the FTP sensor connector terminal and the circuit splice. After repairs, go to step 16 .
14. Replace FTP sensor. See FUEL TANK PRESSURE SENSOR under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. After repairs, go to step 16 .
15. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
16. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
17. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The ignition voltage is 6-18 volts.

1. The AF ECM monitors a 5-volt reference voltage that is less than 4 volts or greater than 6 volts.
2. The condition exists for at least 1 second.

1. The AF ECM will illuminate the MIL.
2. The AF ECM will store this DTC as History during one drive trip in which the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The AF ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
3. The History status will clear after 40 consecutive warm-up cycles with no failures of any DTC.
4. All DTC information can be cleared using a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
3. Are both DTC P1635 and P1639 also set in the PCM? If yes, go to step 7 . If no, go to next step.
4. Is DTC P1639 set in the PCM? If yes, go to step 8 . If no, go to next step.
5. Turn OFF the ignition. Disconnect the AF ECM. Turn ON the ignition, with the engine OFF. Measure the voltage on the 5-volt reference signal circuit with a DMM. Does the DMM display a voltage within 4.8 V-5.2 V? If yes, go to step 11 . If no, go to next step.
6. Test the 5-volt reference circuit between the AF ECM and the PCM for an open condition. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 12 .
7. Turn OFF the ignition. Disconnect the AF ECM. Turn ON the ignition, with the engine OFF. Measure the voltage on the 5-volt reference signal circuit with a DMM. Does the DMM display a voltage above 5.2 V? If yes, go to step 9 . If no, go to step 13 .
8. Turn OFF the ignition. Disconnect the AF ECM. Turn ON the ignition, with the engine OFF. Measure the voltage on the 5-volt reference signal circuit with a DMM. Does the DMM display a voltage within 4.8 V-5.2 V? If yes, go to step 13 . If no, go to step 10 .
9. Test the following 5-volt reference circuits for a short to battery voltage. AF ECM 5-volt reference signal. Gasoline Fuel Tank Pressure (FTP) sensor. Throttle Position (TP) sensor. Gasoline fuel level signal. MAP sensor. EGR sensor. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 14 .
10. Test the following 5-volt reference circuits for a short to ground. AF ECM 5-volt reference signal. Gasoline Fuel Tank Pressure (FTP) sensor. TP sensor. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 12 .
11. Inspect for poor connections at the harness connectors of the AF ECM. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 13 .
12. Inspect for poor connections at the harness connectors of the PCM. Did you find and correct the condition? If yes, go to step 15 . If no, go to step 14 .
13. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to step 15 .
14. Replace the PCM. See «POWERTRAIN CONTROL MODULE»(ref-150199-S07873139252002110500000) under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - GASOLINE article. After repairs, go to next step.
15. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
16. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.

The ignition voltage is 6-18 volts.

1. The AF ECM detects that the commanded state of the driver and the actual state of the control circuit do not match.
2. All the above conditions are present for a minimum of 2 seconds.

1. The AF ECM will not illuminate the MIL.
2. The AF ECM will store this DTC as History during one drive trip in which the diagnostic runs and fails.
3. The AF ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

1. The Last Test Failed status will clear when the DTC runs and passes.
2. The History status will clear after 40 consecutive warm-up cycles with no failures of any DTC.
3. All DTC information can be cleared using a scan tool.

1. Perform diagnostic system check - engine controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-151060-S15412498342002120600000) under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
2. Turn ON the ignition, with the engine OFF. With a scan tool, command the AF fuel pump relay ON and OFF. Does the AF fuel pump relay turn ON and OFF with each command? If yes, go to next step. If no, go to step 4 .
3. Observe the Freeze Frame and/or the Failure records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You also may operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure Records data. Observe the DTC information from the AF ECM and PCM. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see «INTERMITTENT CONDITIONS»(ref-151060-S11013523522002120600000) under SELF-DIAGNOSTIC SYSTEM.
4. Turn OFF the ignition. Disconnect the AF fuel pump relay. Turn ON the ignition, with the engine OFF. Probe the control circuit of the AF fuel pump relay with a test lamp connected to battery voltage. With a scan tool, command the AF fuel pump relay ON and OFF. Does the test lamp turn ON and OFF with each command? If yes, go to next step. If no, go to step 6 .
5. Connect a test lamp between the control circuit of the AF fuel pump relay and the coil power circuit of the relay. With a scan tool, command the AF fuel pump relay ON and OFF. Does the test lamp turn ON and OFF with each command? If yes, go to step 9 . If no, go to step 11 .
6. Does the test lamp remain illuminated with each command? If yes, go to step 8 . If no, go to next step.
7. Test the control circuit of the fuel pump relay for a short to power or an open. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 10 .
8. Test the control circuit of the fuel pump relay for a short to ground. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 10 .
9. Inspect for poor connections at the AF fuel pump relay. Did you find and correct the condition? If yes, go to step 14 . If no go to step 12 .
10. Inspect for poor connections at the harness connectors of the AF ECM. Did you find and correct the condition? If yes, go to step 14 . If no, go to step 13 .
11. Repair the coil power circuit of the relay. After repairs, go to step 14 .
12. Replace the AF fuel pump relay. After repairs, go to step 14 .
13. Replace the AF ECM. See ALTERNATIVE FUELS ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAB & CHASSIS, SIERRA & SILVERADO - BI-FUEL & CNG article. Program (flash) AF ECM with correct vehicle software and calibration. See «ALTERNATIVE FUEL ENGINE CONTROL MODULE»(ref-151060-S07067625962002120600000) under PROGRAMMING. After performing repairs and procedures, go to next step.
14. Using scan tool, clear DTCs. Start engine. Allow engine to idle until engine reaches normal operating temperature. Select DTC and Specific DTC function. Enter DTC number which was set. Operate vehicle within the Conditions For Setting DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2 .
15. With a scan tool, select Capture Info in order to observe the stored information. Does the scan tool display any DTCs that you have not diagnosed? If yes, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . If no, system is okay.