Engine Controls - Tests W/codes - 4.3l Chevrolet Pickup C1500

INTRODUCTION

Most engine control problems result from mechanical failures, poor electrical connections or damaged vacuum hoses. Before condemning the computer system, perform checks and inspections covered in appropriate F - GASOLINE BASIC TESTING article in this section. See list below. Failure to do so may result in lost diagnostic time.

1. F - BASIC TESTING (for Astro & Safari)
2. F - BASIC TESTING (for "S" & "T" Series)
3. F - BASIC TESTING - GASOLINE (for "C" & "K" Series)
4. F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

If no faults were found while performing BASIC TESTING PROCEDURES, proceed with DIAGNOSTIC PROCEDURE. If no fault codes or only a non-running Code 12 is present and driveability problems exist, proceed to appropriate TESTS W/O CODES article in this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.). If only intermittent codes are present, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section. See list below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

TERMINOLOGY

Due to Federal government requirements, manufacturers may use names and acronyms for systems and components different than those used in previous years. The following table will help eliminate confusion when dealing with these components and systems. Only relevant components and systems whose names have changed from current General Motors Corp. terminology have been listed.

SAE TERMINOLOGY

SELF-DIAGNOSTIC SYSTEM

All vehicle are equipped with either an Electronic Control Module (ECM), Powertrain Control Module (PCM) or Vehicle Control Module (VCM). Unless specifically stated, references to ECM also apply to PCM or VCM equipped vehicles.

The ECM is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, ECM will illuminate the SERVICE ENGINE SOON light located on instrument panel. This light is also referred to as the Malfunction Indicator Light (MIL). When malfunction is detected and MIL is turned on, a corresponding trouble code will be stored in ECM memory. To retrieve stored codes, see READING TROUBLE CODES or RETRIEVING CODES (NON-SCAN) . Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.

Hard Failures

Hard failures cause MIL to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause must be found using diagnostic (code) charts. If a sensor fails, control unit will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but driveability can be poor.

Intermittent Failures

Intermittent failures cause MIL to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in ECM memory. If related fault does not reoccur within 50 engine restarts, it will be erased from ECM memory. Intermittent failures may be caused by faulty sensor, connector or wiring. See INTERMITTENTS in appropriate TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

DIAGNOSTIC PROCEDURE

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

1. Ensure all engine systems not related to the computer are operating properly. DO NOT proceed with testing unless all other problems have been repaired. Perform diagnostic circuit check before using trouble code charts. See appropriate BASIC TESTING article in this section below. F - BASIC TESTING (for Astro & Safari) F - BASIC TESTING (for "S" & "T" Series) F - BASIC TESTING - GASOLINE (for "C" & "K" Series) F - BASIC TESTING - GASOLINE (For "G" & "P" Series)
2. If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes cause Malfunction Indicator Light (MIL) to illuminate continuously with engine running. See «HARD OR INTERMITTENT TROUBLE CODE DETERMINATION»(/chevrolet/pickup-c1500/1988-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-43l__hard-or-intermittent-trouble-code-determination) . For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, proceed to INTERMITTENTS in appropriate TESTS W/O CODES article in this section below. Exceptions are Code 13, 15, 24, 44 and 45 charts, which can help diagnose intermittent codes. H - TESTS W/O CODES (for Astro & Safari) H - TESTS W/O CODES (for "S" & "T" Series) H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series) H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)
3. If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in appropriate TESTS W/O CODES article in this section above. From there you will be sent to the appropriate area in appropriate I - SYS/COMP TESTS article in this section below. I - SYSTEM/COMPONENT TESTS (for Astro & Safari) I - SYSTEM/COMPONENT TESTS (for "S" & "T" Series) I - SYSTEM/COMPONENT TESTS - GASOLINE (for "C" & "K" Series) I - SYSTEM/COMPONENT TESTS - GASOLINE (For "G" & "P" Series)
4. After repairs are made, clear trouble codes and perform FIELD SERVICE MODE CHECK in appropriate BASIC TESTING article in this section below. F - BASIC TESTING (for Astro & Safari) F - BASIC TESTING (for "S" & "T" Series) F - BASIC TESTING - GASOLINE (for "C" & "K" Series) F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

RETRIEVING CODES (NON-SCAN)

Note. The Assembly Line Data Link (ALDL) connector may also be referred to as the Data Link Connector (DLC) in flow charts. This is the same connector.

1. Turn ignition on with engine off. Malfunction Indicator Light (MIL) should glow. Locate Data Link Connector (DLC), attached to ECM wiring harness. Most DLCs are located under dash on driver's side of vehicle. For exact location of DLC, see appropriate COMPONENT LOCATIONS illustration in appropriate I - SYS/COMP TESTS article in this section below. Turn ignition on. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of DLC. (Scheme 3) I - SYSTEM/COMPONENT TESTS (for Astro & Safari) I - SYSTEM/COMPONENT TESTS (for "S" & "T" Series) I - SYSTEM/COMPONENT TESTS - GASOLINE (for "C" & "K" Series) I - SYSTEM/COMPONENT TESTS - GASOLINE (For "G" & "P" Series) NOTE: Inserting jumper wire into test and ground terminals of DLC with engine running will cause fuel-injected vehicles to enter field service mode and codes will not flash. See FIELD SERVICE MODE in appropriate BASIC TESTING article in the ENGINE PERFORMANCE section below. F - BASIC TESTING (for Astro & Safari) F - BASIC TESTING (for "S" & "T" Series) F - BASIC TESTING - GASOLINE (for "C" & "K" Series) F - BASIC TESTING - GASOLINE (For "G" & "P" Series)
2. Malfunction Indicator Light (MIL) should flash codes. Each code is flashed 3 times. If codes do not flash, perform DIAGNOSTIC CIRCUIT CHECK in appropriate BASIC TESTING article in this section above. To exit diagnostic mode, turn ignition off and remove jumper wire from DLC.

Scheme 3

READING TROUBLE CODES

Note. Trouble codes retrieved from ECM/PCM/VCM may be related to either engine or transmission. For engine-related codes, use this article. For transmission-related codes, see appropriate G - ELECTRONIC TRANSMISSION TESTS W/ CODES article in the ENGINE PERFORMANCE section below. To identify whether codes relate to transmission or engine, see TROUBLE CODE IDENTIFICATION table.

1. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for Astro & Safari)
2. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "S" & "T" Series)
3. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "C" & "K" Series)
4. AUTO TRANS DIAGNOSIS - 4L60-E & 4L80-E (For "G" & "P" Series)

The ECM stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Read trouble codes by counting Malfunction Indicator Light (MIL) flashes or with diagnostic scan tester connected to the Data Link Connector (DLC). The tester is faster, and capable of reading information which would require testing individual ECM and sensor/solenoid connector terminals with a digital voltmeter. See SCAN TESTER DATA table and SCAN TESTER USAGE .

Note. When using a scan tester, there is a time delay between serial data updates. For instantaneous response, a digital voltmeter must be used.

If scan tester is not available, MIL flashes can be read by grounding DLC terminal with ignition on and engine off. For example, FLASH, FLASH, pause, FLASH, longer pause, indicates Code 21. The first series of flashes are the first digit of trouble code. The second series of flashes are the second digit of trouble code. Trouble codes are displayed starting with the lowest code. Each code is displayed 3 times and will continue as long as DLC is grounded.

Note. Trouble codes will be recorded at various operating times. Some codes require sensor or switch operation for 5 seconds and others may require longer under certain conditions. Some codes may not set in a service bay operational mode.

TROUBLE CODE ID (EXCEPT 4.3L M/T "S" & "T" SERIES PICKUP)

1. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for Astro & Safari)
2. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "S" & "T" Series)
3. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "C" & "K" Series)
4. AUTO TRANS DIAGNOSIS - 4L60-E & 4L80-E (For "G" & "P" Series) TROUBLE CODE ID (4.3L "S" & "T" SERIES PICKUP M/T) Code Probable Cause P0107 MAP Signal Voltage Low (4.3L) P0108 MAP Signal Voltage High (4.3L) P0122 TP Sensor Circuit Signal Voltage Low P0123 TP Sensor Circuit Signal Voltage High P0131 HO2S Circuit Low Signal Voltage P0132 HO2S Circuit High Signal Voltage P0139 HO2S Circuit Open (4.3L) P0171 Fuel Trim Lean (Lean Exhaust) P0172 Fuel Trim Rich (Rich Exhaust) P0320 Ignition Control Error (4.3L) P0321 Spark Reference Circuit P0327 Knock Sensor Circuit Grounded (4.3L) P0328 Knock Sensor Circuit Open (4.3L) P0342 Cam Signal Circuit P0400 EGR Error P0500 VSS Error (4.3L) P1222 Fuel Pump Voltage Low P1530 A/C Head Pressure Switch Circuit P1531 Low A/C Refrigerant Charge P1558 Cruise System Problem P1561 Cruise Vent Solenoid Circuit P1562 Cruise Vacuum Solenoid Circuit P1565 Cruise Servo Position Sensor Circuit P1567 Cruise Switches Circuit P1568 Cruise System Problem P1571 (1) Traction Control Torque Out Of Range P1573 (1) Traction Control Fault (Loss Of ABS/TCS Serial Data) P1599 Cruise Power Management P1623 PROM Error P1632 System Voltage High (4.3L) (1) Transmission code. For automatic transmission code diagnosis, see appropriate G - ELECTRONIC TRANSMISSION TESTS W/ CODES article in this section. See bullet list following this table.
5. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for Astro & Safari)
6. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "S" & "T" Series)
7. G - TESTS W/CODES - ELECTRONIC TRANSMISSION (for "C" & "K" Series)

Note. Trouble code charts should only be used if Malfunction Indicator Light (MIL) is illuminated (indicating a current problem exists). Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes. Anytime Code 51, 52 or 55 is displayed with another code, start with 50-series code first and proceed to lower numbered codes.

HARD OR INTERMITTENT TROUBLE CODE DETERMINATION

During any diagnostic procedure, determine if codes are due to hard or intermittent failure. Diagnostic charts will not usually help diagnose intermittent codes. To determine hard codes and intermittent codes, proceed as follows

1. MANUALLY enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode and clear trouble codes. See «CLEARING TROUBLE CODES»(/chevrolet/pickup-c1500/1988-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-43l__clearing-trouble-codes) .
2. Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. MIL should go out. Run warm engine at specified curb idle for 2 minutes and note MIL.
3. If MIL comes on, manually enter diagnostic mode. Read and record trouble codes. This reveals hard failure codes. Codes 13, 15, 24, 44, 45 and 55 may require a road test to reset hard failure after trouble codes were cleared.
4. If MIL does not come on, all stored trouble codes were intermittent failures. Exceptions are noted under DIAGNOSTIC PROCEDURE.

CLEARING TROUBLE CODES

Turn ignition switch to ON position and ground diagnostic test terminal "B" at DLC. (Scheme 3) Turn ignition switch to OFF position and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead. Codes may also be cleared using the General Motors Tech 1 scan tester.

ECM LOCATION

For ECM locations, see appropriate COMPONENT LOCATIONS illustration in appropriate I - SYS/COMP TESTS article in this section as listed below.

1. I - SYSTEM/COMPONENT TESTS (for Astro & Safari)
2. I - SYSTEM/COMPONENT TESTS (for "S" & "T" Series)
3. I - SYSTEM/COMPONENT TESTS - GASOLINE (for "C" & "K" Series)
4. I - SYSTEM/COMPONENT TESTS - GASOLINE (For "G" & "P" Series)

Diagnostic Aids

Diagnostic aids (located in many trouble code charts) are provided as additional tips to help with diagnosis when inspected circuit is okay.

Field Service Mode Check

Malfunction Indicator Light (MIL) indicates operational mode of engine if DLC is grounded while engine is running. Light response confirms proper fuel system operation and verifies closed loop operation. Clear codes and perform this test after any repair is completed. Field service mode check can be found by proceeding to FIELD SERVICE MODE CHECK in appropriate BASIC TESTING article in this section as listed below.

1. F - BASIC TESTING (for Astro & Safari)
2. F - BASIC TESTING (for "S" & "T" Series)
3. F - BASIC TESTING - GASOLINE (for "C" & "K" Series)
4. F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

SPECIAL TOOLS (DIAGNOSTIC)

Note. A special scan tester, plugged into the DLC, can read trouble codes, check system voltages on the serial data line and save a great deal of time. For additional information, see tester owner's manual. Also, see SCAN TESTER USAGE and SCAN TESTER DATA .

The computerized engine control system is most easily diagnosed using a scan tester. However, other tools may aid in diagnosing problems if a scan tester is unavailable. These tools are a tachometer, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connector at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.

SCAN TESTER USAGE

Note. Before connecting scan tester, check diagnostic system and ensure accurate information is received by scan tester. Perform DIAGNOSTIC CIRCUIT CHECK in appropriate BASIC TESTING article in this section below. If vehicle does not pass diagnostic circuit check, information received by scan tester may be invalid.

1. F - BASIC TESTING (for Astro & Safari)
2. F - BASIC TESTING (for "S" & "T" Series)
3. F - BASIC TESTING - GASOLINE (for "C" & "K" Series)
4. F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

The scan tester is a specialized tester which can diagnose on-board computer control systems by providing almost instant access to circuit voltage information without crawling under dash or hood to backprobe sensors and connectors. scan testers reduce diagnostic time by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN TESTER DATA table.

Scan testers also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication output signals have been sent to devices by the ECM. They do not indicate whether devices respond properly to that signal. This must be verified at output device using a voltmeter or test light.

Note. Code 12 should always exist when DLC test terminal is grounded with key on and engine off, but it may not be indicated by all makes of scan tester.

If trouble codes are not present, a problem may still exist. Driveability-related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Out-of-calibration sensors WILL NOT set a trouble code, but WILL cause driveability problems. A scan tester is the easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See SCAN TESTER DATA table.

Note. Information obtained by scan tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off and remove tester. Turn ignition on and ground DLC test terminal. If same codes are not flashed by Malfunction Indicator Light (MIL) as were indicated by scan tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.

SCAN TESTER DATA

Note. Information contained in the following table is typical of readings taken on vehicle with engine idling, upper radiator hose hot, throttle closed, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Not all devices and systems are used on all models. For additional information, see tester owner's manual.

SCAN TESTER DATA

TROUBLE CODE CHARTS - 4.3L (EXC M/T "S" & "T" SERIES PICKUP)

Note. The following diagnostic mini-schematics are supplied courtesy of General Motors Corp.

CODE 13, OPEN OXYGEN SENSOR CIRCUIT

When exhaust temperature is less than 600°F (316°C), O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not allow system to enter closed loop. On 4.3L engines, oxygen sensor is equipped with an internal heating unit. This allows sensor to reach operating temperature quicker and maintain closed loop operation even during extended idle. Heating element resistance should be 3.5-14 ohms at 662°F (350°C).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 13 will set at normal operating temperature if at least 2 minutes have passed since engine start, Code 21 or 22 is not present, O2 signal voltage is steady at .35-.55 volt and throttle position sensor signal is greater than idle. All conditions must be met for at least one minute.
2. This determines if fault is in O2 sensor, ECM or wiring.
3. Use only a high-impedance Digital Volt-Ohmmeter (DVOM) while checking for continuity in signal and ground circuits. If ground circuit is open, voltage on signal circuit will be greater than .6 volt.

Verify a clean, tight connection for the sensor ground. An open circuit at sensor signal terminal or ground terminal will result in a Code 13.

CODE 13 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 13 Schematic (4.3L) Open Oxygen Sensor Circuit. Scheme 4

Code 13 Flow Chart (4.3L) Open Oxygen Sensor Circuit. Scheme 5

CODE 14, COOLANT SENSOR SIGNAL VOLTAGE LOW

Coolant temperature sensor input is used to determining control of fuel delivery, engine timing, idle speed and converter clutch (TCC) application. As engine warms, sensor resistance reduces. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at coolant sensor signal terminal.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This tests if code was set because of a hard failure or intermittent condition. Code 14 sets if signal voltage indicates a coolant temperature greater than 275°F (135°C) for more than 6 seconds.
2. This simulates conditions for a Code 15. If scan tester displays a low temperature, ECM and wiring are not at fault.

After engine is started, temperature should rise steadily to about 194°F (90°C), then stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature sensor and MAT sensor (if equipped) should read close to each other, when measured with a scan tester.

CODE 14 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 14 Schematic (4.3L) Coolant Sensor Signal Voltage Low. Scheme 6

Code 14 Flow Chart (4.3L) Coolant Sensor Signal Voltage Low. Scheme 7

CODE 15, COOLANT SENSOR SIGNAL VOLTAGE HIGH

As engine warms, sensor resistance reduces and voltage drops. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at ECM coolant sensor signal terminal. If sensor signal circuit opens, ECM will see -56°F (-49°C) and deliver fuel for this temperature.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if code was set as a result of a hard failure or intermittent condition. Code 15 will set if engine is running for more 50 seconds and signal voltage indicates a coolant temperature less than -22°F (-30°C) for more than 30 seconds.
2. This simulates conditions for a Code 14. If ECM recognizes grounded circuit and displays a high temperature, ECM and wiring are okay.
3. This determines if problem is ECM or wiring. There should be 5 volts present at sensor when measured with a DVOM.

After engine starts, temperature should rise steadily to about 194°F (90°C) and stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature sensor and MAT sensor (if equipped) should read close to each other when measured with a scan tester. Code 15 will also set if sensor signal or ground circuit is open.

CODE 15 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 15 Flow Chart (4.3L) Coolant Sensor Signal Voltage High. Scheme 8

CODE 16, VSS BUFFER FAULT - "G", "L", "M", "S" & "T" SERIES

Note. Test number refers to number on diagnostic chart.

1. Checks for battery voltage at VSS buffer.
2. Tests for proper ground path for VSS buffer
3. Tests for vss buffer signal to PCM
4. Tests for faulty connections and a faulty VSS buffer

Check for poor connections or damaged harness.

Code 16 Schematic ("G" Series) VSS Buffer Fault. Scheme 9

Code 16 Schematic ("L" & "M" Series) VSS Buffer Fault. Scheme 10

Code 16 Schematic ("S" & "T" Series A/T & Util. Vehicles M/T) VSS Buffer Fault. Scheme 11

Code 16 Flow Chart VSS Buffer Fault. Scheme 12

CODE 21, TPS SIGNAL VOLTAGE HIGH

Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from about .50 volt at idle to 4 volts at wide open throttle. Each time TPS voltage drops to less than 1.25 volts and stops, ECM assumes this is zero degrees throttle angle and measures throttle percentage angle from this point.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 21 and checks if fault is a hard failure or an intermittent condition. Code 21 will set if TPS voltage is greater than 2.5 volts 2-10 seconds with engine running.
2. This test simulates conditions for Code 22. If ECM recognizes low voltage signal and sets Code 22, ECM and power and signal circuits are not at fault.
3. This step isolates a faulty sensor, ECM or an open ground circuit.

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 21 will also result if ground circuit is open or TPS signal circuit is shorted to voltage.

CODE 21 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 21 Schematic (4.3L) TPC Signal Voltage High. Scheme 13

Code 21 Flow Chart (4.3L) TPC Signal Voltage High. Scheme 14

CODE 22, TPS SIGNAL VOLTAGE LOW

Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from less than about .50 volt at idle to 4 volts at wide open throttle.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 22 and tests if fault is a hard failure or an intermittent condition. Code 22 will set if engine is running and TPS voltage is less than .2 volt for 2-4 seconds.
2. This simulates Code 21. If ECM recognizes a high voltage signal and sets Code 21, ECM and wiring are not at fault. Replace TPS.
3. This simulates a high voltage signal to check for on open TPS signal circuit.

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.0 volt. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.

CODE 22 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 22 Flow Chart (4.3L) TPS Signal Voltage Low. Scheme 15

CODE 23, INTAKE AIR TEMPERATURE LOW

ECM supplies and monitors a voltage signal (4-6 volts) to sensor. When temperatures are low, sensor resistance is high and ECM will see a high-monitored voltage signal. As temperature increases, sensor resistance decreases and voltage sensed by ECM drops.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if Code 23 is a hard failure or an intermittent condition. Code 23 will set if engine is running for one minute, MAT sensor temperature is less than -31°F (-35°C) for 12 seconds and speed sensor signal is not present.
2. This simulates conditions for a Code 25. If scan tester displays a high temperature, ECM and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits.

If engine is allowed to cool overnight, coolant and MAT sensors should read close to each other, when measured with a scan tester. A Code 23 will also result if signal and ground circuits become open.

CODE 23 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 23 Schematic (4.3L) Intake Air Temperature Low. Scheme 16

Code 23 Flow Chart (4.3L) Intake Air Temperature Low. Scheme 17

CODE 24, VSS - "P" SERIES M/T & "S" & "T" SERIES UTIL VEH M/T

VSS output sensor is a magnetic induction type. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning. Since vehicle speed is taken from transfer case on 4WD vehicles, output speed sensor signal on these units goes directly to PCM. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at least 3000 RPM and output speed is less than 250 RPM for at least 1.5 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies VSS voltage at PCM.
2. Test checks VSS circuit at buffer module.
3. Test verifies VSS signal at sensor.

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section below. While Code 24 is set, scan tester will display an RPM derived from input speed. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

Code 24, Schematic ("P" Series M/T) Vehicle Speed Sensor. Scheme 18

Code 24, Schematic ("S" & "T" Series Utility Vehicles M/T) Vehicle Speed Sensor. Scheme 19

Code 24, Flow Chart Vehicle Speed Sensor. Scheme 20

CODE 24, VSS - 4.3L "T" SERIES PICKUP M/T

Note. The VSS buffer is an internal part of the Digital Ratio Adapter Controller (DRAC) and the terms are used interchangeably.

ECM applies and monitors 12 volts on VSS signal circuit. Circuit is connected to VSS buffer which alternately grounds this circuit when it is receiving voltage pulses from Vehicle Speed Sensor (VSS). Scan tester reading should closely match speedometer reading.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks to see if code is set as a result of a hard failure or an intermittent condition.
2. This determines if DRAC is receiving AC signal from VSS.
3. This test monitors DRAC voltage on circuit No. 437. With wheels turning, pulsing voltage should be present. Voltage variation will be greater at a low speed to an average of 4-6 volts at about 20 MPH.

Scan tester reading should closely match speedometer reading with drive wheels turning. If vehicle is equipped with an automatic transmission, check park/neutral switch adjustment. If no problem is found while using flow chart, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

Code 24 Schematic (4.3L "T" Series Pickup M/T Vehicle Speed Sensor. Scheme 21

Code 24 Flow Chart Vehicle Speed Sensor. Scheme 22

CODE 24, VSS TRANSMISSION OUTPUT SIGNAL - "G", "L" & "M" SERIES, "P" SERIES M/T & "S" & "T" SERIES A/T

VSS output sensor is a magnetic induction type. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning. Since vehicle speed is taken from transfer case on 4WD vehicles, output speed sensor signal on these units goes directly to PCM. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at least 3000 RPM and output speed is less than 250 RPM for at least 1.5 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies VSS voltage at PCM.
2. Test checks VSS buffer ground circuit.
3. Test checks VSS circuit at buffer module.
4. Test verifies VSS signal at sensor.

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section below. While Code 24 is set, scan tester will display an RPM derived from input speed. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

Code 24 Schematic ("G" Series & "S" & "T" Series A/T) Vehicle Speed Sensor Transmission Output Signal. Scheme 23

Code 24 Schematic ("L" & "M" Series) Vehicle Speed Sensor Transmission Output Signal. Scheme 24

Code 24 Schematic ("P" Series M/T) Vehicle Speed Sensor Transmission Output Signal. Scheme 25

Code 24 Flow Chart Vehicle Speed Sensor Transmission Output Signal. Scheme 26

CODE 25, INTAKE AIR TEMPERATURE HIGH

ECM applies and monitors a voltage signal (4-6 volts) to MAT sensor. When manifold air is cold, sensor resistance is high and ECM sees a high signal voltage. As air warms, resistance decreases and voltage sensed by ECM drops. Sensor resistance can be measured at sensor terminals with harness disconnected.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if code is a hard failure or an intermittent condition. Code 25 will set if a VSS signal is present (2.5L) and monitored MAT sensor temperature is greater than 275°F (135°C).
2. This simulates conditions for a Code 23. If scan tester displays a low temperature, ECM and wiring are not at fault.

If engine is allowed to cool overnight, coolant temperature sensor and MAT sensor should read close to each other, when measured with a scan tester. A Code 25 will also result if sensor signal circuit is shorted to ground.

CODE 25 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 25 Flow Chart (4.3L) Intake Air Temperature High. Scheme 27

CODE 32, EGR CIRCUIT ERROR WITH LINEAR EGR

ECM regulates linear EGR valve to control exhaust gas recirculation by providing a ground control for internal pintle (solenoid). Pintle will pull away from its seat when energized. ECM controls linear EGR valve based upon coolant temperature and throttle position.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks pintle's ability to be commanded to desired position.
2. Checks for voltage to linear EGR valve to verify if problem is in ignition feed circuit.
3. Checks ECM control circuit by jumpering across harness terminals with a test light and grounding ALDL (DLC) test terminal "B" to command EGR on.

Before replacing ECM, use an ohmmeter and check EGR valve resistances. Resistance between terminals "A" and "E" should be 9.5-10.5 ohms. Resistance between terminals "B" and "D" should be greater than 3000 ohms. Resistance between terminals "B" and "C" should start at 700 ohms and increase to about 4000 ohms as pintle is slowly moved inward.

Code 32 Schematic ("C" & "K" Series A/T) EGR Circuit Error With Linear EGR. Scheme 28

Code 32 Schematic ("P" Series Except 7.4L & "S" & "T" Series 4.3L W/ Calif. Emissions) EGR Circuit Error With Linear EGR. Scheme 29

Code 32 Schematic ("G" Series) EGR Circuit Error With Linear EGR. Scheme 30

Code 32 Schematic ("L" & "M" Series & "S" & "T" Series Pickup 4.3L (VIN W) Exc. W/ Calif. Emissions) EGR Circuit Error With Linear EGR. Scheme 31

Code 32 Schematic ("S" & "T" Series 4.3L (VIN Z) Exc. Calif. Emissions) EGR Circuit Error With Linear EGR. Scheme 32

Code 32 Flow Chart EGR Circuit Error With Linear EGR. Scheme 33

CODE 32, EGR CIRCUIT ERROR - "P" SERIES 4.3L M/T

ECM controls a solenoid regulating vacuum to EGR valve. Normally closed solenoid prevents vacuum from passing until it is energized by ECM. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.

Note. Test numbers refer to test numbers on diagnostic chart.

1. When test terminal "B" of ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to EGR valve. Vacuum should hold.
2. When jumper wire is removed from terminal "B", vacuum to EGR valve should bleed through a vent in solenoid and EGR valve should close. Vacuum gauge may or may not bleed off vacuum. However, this does not indicate a problem.
3. Determines if fault lies in electrical control part of system, connector or solenoid.
4. This system uses a negative backpressure EGR valve. Valve should hold vacuum with engine off.
5. When engine is started, backpressure should cause vacuum to bleed off and valve should fully close.

Prior to replacing ECM, check resistance of all ECM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.

Code 32 Schematic ("P" Series 4.3L M/T) EGR Circuit Error. Scheme 34

Code 32 Flow Chart ("P" Series 4.3L M/T) EGR Circuit Error. Scheme 35

CODE 32, EGR CIRCUIT ERROR - MODELS USING MANIFOLD VACUUM, 2-TERMINAL SOLENOID & BACKPRESSURE EGR

PCM controls a solenoid regulating vacuum to EGR valve. Normally closed solenoid prevents vacuum from passing until it is energized by PCM. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.

Note. Test numbers refer to test numbers on diagnostic chart.

1. EGR valve should open when vacuum is applied to solenoid. Vacuum should hold.
2. When Tech 1 energizes EGR solenoid, vacuum to EGR valve should bleed through a vent in solenoid and EGR valve should close. Vacuum gauge may or may not bleed off vacuum. However, this does not indicate a problem.
3. Determines if fault lies in electrical control part of system, connector or solenoid.
4. This system uses a negative backpressure EGR valve. Valve should hold vacuum with engine off.
5. When engine is started, backpressure should cause vacuum to bleed off and valve should fully close.

Prior to replacing PCM, check resistance of all PCM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.

Code 32 Schematic ("C" & "K" Series 4.3L) EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 36

Code 32 Schematic ("G" Series A/T) EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 37

Code 32 Schematic ("P" Series 4.3L) EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 38

Code 32 Flow Chart EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 39

CODE 32, EGR CKT ERROR MODELS USING PORTED VACUUM & EVRV SOL

PCM controls a solenoid that regulates vacuum to EGR valve. The normally closed solenoid prevents vacuum from passing until it is energized by PCM. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.

ECM checks EGR operation when engine speed is greater than 1600 RPM, MAP sensor signal indicates cruise condition and throttle position are constant.

Note. Test numbers refer to test numbers on diagnostic chart.

1. With ignition on and engine off, solenoid should not be energized or allow vacuum to pass to EGR valve. When Tech 1 energizes solenoid, vacuum should pass through solenoid to EGR valve. Vacuum should hold.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
3. Vehicle must be driven during this test to produce sufficient load to operate EGR. Lightly accelerating (approximately 1/4 throttle) will produce a large and stable enough reading to determine if ECM is commanding system on.

Prior to replacing PCM, check resistance of all PCM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms. EVRV solenoid cannot be checked with a DVOM due to solid state circuitry.

Code 32 Schematic ("C" & "K" Series) EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 40

Code 32 Schematic ("G" Series) EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 41

Code 32 Schematic ("S" & "T" Series 4.3L Pickup A/T & 4.3L Utility M/T) EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 42

Code 32 Flow Chart EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 43

CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If MAP sensor fails, ECM will substitute a fixed MAP value and use TPS input to control fuel delivery.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 33 and determines if it is a hard failure or an intermittent condition. Code 33 will set when voltage signal reading is too high and TPS voltage indicates throttle is closed.
2. This step simulates conditions for a Code 34. If ECM recognizes and indicates low MAP signal, ECM and 5-volt reference and MAP signal circuits are not at fault.

With ignition switch in ON position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage is high. Comparing BARO readings from a known good vehicle using the same sensor is a good way to check accuracy of suspected sensor. Readings should be within .4 volt of each other. Code 33 will also result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.

CODE 33 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 33 (4.3L) MAP Sensor Signal Voltage High. Scheme 44

Code 33 Flow Chart (All Models) MAP Sensor Signal Voltage High. Scheme 45

CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If MAP sensor fails, ECM will substitute a fixed MAP value and use TPS input to control fuel delivery.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This confirms Code 34 and determines if code was a hard failure or an intermittent condition. Code 34 will set when ignition is on and MAP signal voltage is low. On some systems, engine must be running to set code.
2. Jumpering harness 5-volt reference circuit and MAP signal circuit terminals will determine if problem is sensor, ECM or wiring. If ECM recognizes and indicates high MAP signal, ECM and wiring are okay.
3. Scan tester may not display 12 volts. The important thing is that the ECM recognizes voltage as greater than 4 volts (high MAP voltage signal), indicating ECM and MAP signal circuit are not at fault.

With ignition switch in ON position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage will be high. Comparing BARO readings with a known good vehicle using the same sensor is a good way to check accuracy of suspected sensor. Readings should be within .4 volt of each other. A Code 34 will also result if 5-volt reference and MAP signal circuits are open or shorted to ground. If 5-volt reference circuit is not shorted to ground and a Code 22 is stored, check MAP signal circuit for short to ground.

CODE 34 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 34 Flow Chart (All Models) MAP Sensor Signal Voltage Low. Scheme 46

CODE 35, IDLE SPEED ERROR

Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. IAC driver is used to extend and retract IAC valve. Movement is verified by changing engine speed. If no engine speed change occurs, valve can be retested when removed from throttle body.
2. Checks IAC movement quality from step 1). Between 700-1500 RPM, engine speed should change smoothly with each tester light flash while extending or retracting. If IAC valve is retracted beyond control range (about 1500 RPM), it may take many flashes in extend position before engine speed reduces. This is normal on some engines. Fully extending IAC may cause engine to stall. This may be normal.
3. Steps 1) and 2) verified proper IAC valve operation. This step checks IAC circuits. Each light on node light should flash Red and Green, while IAC valve is cycled. While color sequence is not important, if either light is off or does not flash Red and Green, check circuits beginning with poor terminal contacts.

IAC Valve Reset Procedure

Turn ignition off for 10 seconds. Start and run engine for 5 seconds. Turn ignition off another 10 seconds.

A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. Scan counts will be greater than 60 if too low, and zero counts if too high. If idle is too high, stop engine. With ignition on, ground ALDL test terminal "B". Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is greater than 800 RPM, inspect vehicle for vacuum leaks.

System Too Lean

If air/fuel ratio is too lean, idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary and disconnecting IAC may not help. Scan tester and/or digital voltmeter (10 megohm) will read an oxygen sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in fuel.

System Too Rich

If air/fuel ratio is too rich, idle speed will be too low and scan tester counts will usually be greater than 80. The system may be obviously rich with Black smoke from tailpipe. Scan tester and/or voltmeter will read an oxygen sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure or leaking/sticky injectors. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging bore. A silicone-contaminated oxygen sensor will produce lean air/fuel mixture. Oxygen sensor output would be fixed greater than 800 mv (.8 volt). This may also set Code 45.

Throttle Body

Remove IAC and inspect bore for evidence of IAC valve dragging.

IAC Valve Connections

Carefully inspect connections for looseness or corrosion.

PCV Valve

The wrong PCV valve may cause incorrect idle speed.

CODE 35 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 35 Schematic (4.3L) Idle Speed Error. Scheme 47

Code 35 Flow Chart (All Models) Idle Speed Error. Scheme 48

CODE 36, IDLE SPD ACTUATOR FAULT - "C", "K" & "P" SERIES A/T

Control module grounds solenoid circuit when coolant temperature is within range to enable solenoid, allowing vacuum to retract solenoid actuator. This lowers idle speed to controlled IAC idle specification.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks if vacuum source is present.
2. Checks to see if actuator is commanded on.
3. Checks for power, ground and proper connections at actuator solenoid.
4. Checks for open ground circuit.
5. Checks for open driver circuit or faulty control module.

If BARO is low, a 96 second reset will result. If Code 36 is set, Code 35 will usually also be set. Repair actuator circuit first. If IAC counts are zero, idle speed control actuator will disengage and raise idle speed.

Code 36 Schematic ("C" & "K" Series) Idle Speed Actuator Fault. Scheme 49

Code 36 Schematic ("P" Series) Idle Speed Actuator Fault. Scheme 50

Code 36 Flow Chart Idle Speed Actuator Fault. Scheme 51

CODE 42 - IGNITION CONTROL

Code 42 indicates ECM has seen an open or short to ground in the Ignition Control (IC) or by-pass circuits.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
2. This tests for a normal IC ground path through ignition module. If circuit is shorted to ground, reading will be less than 500 ohms.
3. As test light voltage touches by-pass circuit, module should switch. This causes ohmmeter to "over-range" with meter in 100-200 ohm range. A higher ohm range will indicate over 5000 ohms. This test assures module switched.
4. If module did not switch, this step tests for a short in IC circuit, an open in by-pass circuit and a faulty ignition module connection or module.
5. This step confirms Code 42 is a faulty ECM and not an intermittent problem in IC and by-pass circuits.

The scan tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

Code 42 Schematic ("L" & "M" Series 4.3L VIN Z) Ignition Control. Scheme 52

Code 42 Schematic ("P" Series A/T) Ignition Control. Scheme 53

Code 42 Schematic ("G" Series) Ignition Control. Scheme 54

Code 42 Schematic ("P" Series M/T) Ignition Control. Scheme 55

Code 42 Schematic ("L" & "M" Series 4.3L VIN W) Ignition Control. Scheme 56

Code 42 Schem.("S" & "T" Series 4.3L A/T& Util. Vehicles M/T) Ignition Control. Scheme 57

Code 42 Schematic ("T" Series 4.3L Pickup M/T) Ignition Control. Scheme 58

Code 42 Flow Chart Ignition Control. Scheme 59

CODE 43, KNOCK SENSOR (DUAL SENSORS) - "S" & "T" SERIES

Note. Test numbers refer to test numbers on diagnostic chart.

1. Tests for 5-volt signal applied by PCM.
2. An improperly installed sensor can prevent knock sensor from grounding to block.

Control module applies and monitors a 5-volt DC signal to knock sensors. Internal knock sensor circuitry pulls this DC signal down to about 2.5 volts. When knock sensor detects detonation, it generates an AC signal which rides back on DC signal to VCM. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic ("S" & "T" Series) Knock Sensor (Dual Sensors). Scheme 60

Code 43 Flow Chart ("S" & "T" Series) Knock Sensor. Scheme 61

CODE 43, KNOCK SENSOR W/O SPARK MODULE (SINGLE SENSOR) - "C", "G", "K" & "P" SERIES A/T

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 43 will set when vehicle reaches normal operating temperature (but not overheating), high engine load is indicated by MAP sensor and voltage on circuit No. 496 is greater than 3.5 volts DC or less than 1.5 volts DC. This step determines if system is functioning properly at current time.
2. This step determines state of 5-volt reference signal applied to sensor.
3. Checks knock sensor internal resistance.

ECM/PCM applies and monitors a 5-volt DC signal to knock sensor. Internal knock sensor circuitry pulls this DC signal down to about 2.5 volts. When knock sensor detects detonation, it generates an AC signal which rides back on DC signal to ECM/PCM. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic ("C", "G", "K" & "P" Series A/T) Knock Sensor W/O Spark Module (Single Sensor). Scheme 62

Code 43 Flow Chart ("C", "G", "K" & "P" Series A/T) Knock Sensor W/O Spark Module (Single Sensor). Scheme 63

CODE 43, KNOCK SENSOR WITH SPARK CONTROL MODULE - "C", "K" & "P" SERIES

Note. Test numbers refer to test numbers on diagnostic chart.

1. If conditions for a Code 43 exist, scan tester will display YES. A knock signal should exist at idle unless an internal or system problem exists.
2. Determines if system is functioning. Usually, a knock signal can be made by tapping on exhaust manifold. If knock signal is not made, try tapping on engine block near sensor. On models with automatic transmission, it may be necessary to place gear selector lever in Drive.
3. Because Code 43 sets when signal voltage on spark retard line remains low, this test should cause signal on that line to go high. The 12-volt signal should be seen by ECM as a "no knock" signal if ECM and wiring are okay.
4. This test determines if knock signal is detected on sensor-to-controller line or if ESC module is at fault.
5. If sensor line is routed too close to secondary ignition wires, ESC module may see interference as a knock signal.
6. This checks ground circuit to module. An open ground will cause voltage on monitored line to remain constant at about 12 volts. This would cause Code 43 functional test to fail.
7. This should generate a knock signal to controller. This determines if ESC controller is operating correctly.

Code 43 can be caused by a faulty knock sensor connection at ESC module or ECM. Also, check controller-to-ECM signal line for an open or short to ground.

Code 43 Schematic ("C" & "K" Series) Knock Sensor W/ Spark Control Module. Scheme 64

Code 43 Schematic ("P" Series) Knock Sensor W/ Spark Control Module. Scheme 65

Code 43 Flow Chart ("C", "K" & "P" Series) Knock Sensor W/ Spark Control Module. Scheme 66

CODE 44, LEAN EXHAUST INDICATION

Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600°F (316°C). An open sensor circuit or cold sensor causes "open loop" operation.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 44 sets when O2 sensor signal remains low for a precalibrated period and system is operating in "closed loop".

Using scan tester, observe Block Learn Memory (BLM) value at different RPMs. If Code 44 conditions exist, block learn value will be around 150-172.

O2 Sensor Wire

Wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.

Fuel Contamination

Water, even small amounts, near in-tank fuel pump inlet can reach fuel injector, causing a lean exhaust and setting Code 44.

Fuel Pressure

System will be lean if fuel pressure is low. It may be necessary to monitor fuel pressure while driving vehicle. For fuel pressure checking procedure, see appropriate BASIC TESTING article in this section below.

1. F - BASIC TESTING (for Astro & Safari)
2. F - BASIC TESTING (for "S" & "T" Series)
3. F - BASIC TESTING - GASOLINE (for "C" & "K" Series)
4. F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

Exhaust Leaks

If exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into system and past O2 sensor. Vacuum or crankcase leaks can also cause a lean condition. If Code 44 is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

CODE 44 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 44 Flow Chart (4.3L) Lean Exhaust Indication. Scheme 67

Code 44 Flow Chart. Scheme 68

CODE 45, RICH EXHAUST INDICATION

Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600°F (316°C). An open sensor circuit or cold sensor causes "open loop" operation. Code 45 indicates a rich exhaust and diagnosis should begin with: fuel pressure, leaking injector, HEI shielding, canister purge saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Tests if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is greater than 5 percent, O2 sensor signal at ECM is greater than .75 volt for 60 seconds or more.

Code 45, rich exhaust, is most likely caused by one of the following

Fuel Pressure High

If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see appropriate BASIC TESTING article in this section below. The ECM can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.

1. F - BASIC TESTING (for Astro & Safari)
2. F - BASIC TESTING (for "S" & "T" Series)
3. F - BASIC TESTING - GASOLINE (for "C" & "K" Series)
4. F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

Ignition Ground

If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses picked up by ECM on EST harness reference line. Additional pulses result in a higher than actual engine speed signal. The ECM will increase injector pulse width ("on" time) to match increased RPM signal. Scan tester will show higher than actual RPM, which can help diagnose problem.

Fuel Canister

Charcoal canister fuel saturation will cause a rich air/fuel ratio. If full of fuel, check canister control and hoses.

MAP Sensor

If ECM senses higher than normal manifold pressure (low vacuum) system can go rich. Disconnecting MAP sensor allows ECM to substitute a fixed value for the MAP sensor. If rich condition disappears, replace MAP sensor and continue testing.

TPS

An intermittent TPS output will cause system to operate rich due to a false indication of engine acceleration.

O2 Sensor Contamination

O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a White-powdery coating to cover O2 sensor. The false high signal voltage produced (or low oxygen content sensed) is interpreted by ECM as a rich mixture, causing ECM to set Code 45.

EGR Problem

EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. If Code 45 is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

CODE 45 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 45 Flow Chart (4.3L) Rich Exhaust Indication. Scheme 69

CODE 51, PROM ERROR (FAULTY OR INCORRECT PROM)

Ensure all pins are fully inserted in socket. If okay, replace PROM/MEM-CAL, clear memory and recheck. If Code 51 reappears, replace ECM.

CODE 52, FAULTY CALPAK

Ensure all pins are fully inserted in socket. If okay, replace CALPAK, clear memory and recheck. If Code 51 reappears, replace ECM.

CODE 53, SYSTEM OVERVOLTAGE

This code indicates a basic charging system problem. Code 53 will set when voltage at ECM terminal is greater than 17.1 volts for 2 seconds. Check and repair charging system.

CODE 55, ECM/PCM ERROR

Ensure ECM grounds are good and MEM-CAL is properly latched. If okay, replace ECM/PCM. Clear codes and confirm closed loop operation. Check operation of SERVICE ENGINE SOON light.

CODE 54, FUEL PUMP CIRCUIT

The status of fuel pump signal is monitored by the control module and is used to compensate fuel delivery based on system voltage. Signal is also used to store Code 54 if fuel pump relay is defective or if relay voltage is lost after engine is running. Voltage should be present at fuel pump signal terminal of control module the first 2 seconds after ignition is turned on and anytime reference (RPM) pulses are being received by control module.

Code 54 Schematic ("C" & "K" Series Exc. 5.0L/5.7L M/T Fuel Pump Circuit. Scheme 70

Code 54 Schematic ("G" Series) Fuel Pump Circuit. Scheme 71

Code 54 Schematic ("L" & "M" Series & "S" & "T" Series Utility Vehicles M/T Fuel Pump Circuit. Scheme 72

Code 54 Schematic ("P" Series A/T) Fuel Pump Circuit. Scheme 73

Code 54 Schematic ("P" Series M/T) Fuel Pump Circuit. Scheme 74

Code 54 Schematic ("S" & "T" Series Pickup A/T) Fuel Pump Circuit. Scheme 75

Code 54 Flow Chart Fuel Pump Circuit. Scheme 76

CODE P0107, MAP SENSOR CKT - 4.3L "S" & "T" SERIES PICKUP M/T

The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The VCM receives this information as a signal voltage that varies from 1-1.5 volts at idle to 4-4.6 volts at full throttle. Scan tool displays manifold pressure in volts. Low pressure (high vacuum) reads a low voltage, while high pressure (low vacuum) reads a high voltage. If the MAP sensor fails, VCM will substitute a fixed MAP value and use throttle position sensor to control fuel delivery.

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if code is set because of a hard failure or intermittent condition.
2. Jumping circuits determines if sensor is at fault or if problem exists in VCM or wiring.
3. Tech-1 may not display 5 volts. The important item is that VCM recognized voltage as more than 4 volts, indicating that VCM and circuit are okay.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for

1. Intermittents: An intermittent or open in Gray or Light Green wire circuits will set Code P0107.
2. Sensor Accuracy: With ignition on and engine off, pressure signal is equal to atmospheric pressure with signal high. This information is used by VCM as an indication of altitude. Comparison of this reading with a known good vehicle with same sensor is a way to check accuracy of suspect sensor. Reading should be the same within 0.4 volt.
3. Disconnect sensor from bracket and twist sensor by hand to check for intermittents. Output changes greater than 0.1 volt indicate a bad sensor or connection.

Code P0107 Schematic (4.3L "S" & "T" Series Pickup M/T) MAP Sensor Circuit. Scheme 77

CODE P0108, MAP SENSOR CKT - 4.3L "S" & "T" SERIES PICKUP M/T

The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The VCM receives this information as a signal voltage that varies from 1-1.5 volts at idle to 4-4.6 volts at full throttle. If the MAP sensor fails, VCM will substitute a fixed MAP value and use throttle position sensor to control fuel delivery.

Note. Test numbers refer to numbers on diagnostic chart.

1. Rough, unstable or incorrect idle and low manifold vacuum must be corrected before using chart.
2. This simulates conditions for Code P0108. If VCM recognizes change, VCM and circuits are okay.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for

1. Intermittents: An intermittent or open in wire circuits will set Code P0108.
2. Sensor Accuracy: With ignition on and engine off, pressure signal is equal to atmospheric pressure with signal high. This information is used by VCM as an indication of altitude. Comparison of this reading with a known good vehicle with same sensor is a way to check accuracy of suspect sensor. Reading should be the same within 0.4 volt.
3. Disconnect sensor from bracket and twist sensor by hand to check for intermittents. Output changes greater than 0.1 volt indicate a bad sensor or connection.

CODE P0117, COOLANT TEMP. SENS - "S" & "T" SERIES PICKUP M/T

ECT sensor uses a thermistor to control signal voltage to PCM. PCM applies and monitors a 5 volts to ECT sensor. When engine coolant is cold, sensor (thermistor) resistance is high and PCM will sense a high signal voltage. As engine coolant warms up, sensor resistance becomes less and PCM voltage drops. With Code P0117 set, PCM will turn cooling fans on and use a default engine coolant temperature value based on run time. PCM will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if conditions necessary to set Code P0117 exist.
2. Determines if circuit is shorted to ground.

Tech 1 displays engine temperature in degrees celsius (°C) and fahrenheit (°F). After engine is started, temperature should rise steadily to 194°F (90°C), then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for

1. Poor Connection Or Damaged Harness: Inspect PCM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test: Using Tech 1, monitor engine coolant temperature while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating the location of the malfunction.
3. Shifted Sensor: See TEMPERATURE-TO-RESISTANCE VALUES table to test ECT sensor at various temperature levels to evaluate the possibility of a shifted (mis-scaled) sensor which may result in driveability complaints.
4. A faulty connection, or an open in sensor circuits will cause Code P0117 to set.

TEMPERATURE-TO-RESISTANCE VALUES (1)

Code P0117 Schematic (4.3L "S" And "T" Series Pickup M/T) Coolant Temperature Sensor. Scheme 78

Code P0117 Flow Chart Coolant Temperature Sensor. Scheme 79

CODE P0118, COOLANT TEMP. SENSOR - 4.3L "S" & "T" SERIES PICKUP M/T

ECT sensor uses a thermistor to control signal voltage to PCM. PCM applies and monitors a 5 volts to ECT sensor. When engine coolant is cold, sensor (thermistor) resistance is high and PCM will sense a high signal voltage. As engine coolant warms up, sensor resistance becomes less and PCM voltage drops. With Code P0118 set, PCM will turn cooling fans on and use a default engine coolant temperature value based on run time. PCM will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if conditions necessary to set Code P0118 exist.
2. This test will determine if signal circuit is shorted to ground which will cause conditions for Code P0118.
3. This test will determine if circuit is open. If circuits are okay, check connections at PCM and ECT sensor.

Tech 1 displays engine temperature in degrees celsius (°C) and fahrenheit (°F). After engine is started, temperature should rise steadily to 194°F (90°C), then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check the following for

1. A short to ground in circuit No. 410 (Yellow wire).
2. Intermittent Test: Using Tech 1, monitor engine coolant temperature while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating the location of the malfunction.
3. Shifted Sensor: See TEMPERATURE-TO-RESISTANCE VALUES table to test ECT sensor at various temperature levels to evaluate the possibility of a shifted (mis-scaled) sensor which may result in driveability complaints.

TEMPERATURE-TO-RESISTANCE VALUES (1)

Code P0118 Flow Chart Coolant Temperature Sensor. Scheme 80

CODE P0122, THROTTLE POS SENS - 4.3L "S" & "T" SERIES P/U M/T

The Throttle Position (TP) sensor provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from .20-.74 volt at idle to greater than 4 volts at Wide Open Throttle (WOT). TP sensor signal is one of the most important inputs used by PCM for fuel control and for most of the PCM control outputs.

With Code P0122 set, Tech 1 will continue to display the actual throttle sensor voltage, but ignore TP signal and read mass air flow value for other PCM controlled functions, PCM will not allow 4th gear, TCC or cruise control operation, 1st, 2nd, and 3rd allowed. PCM will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if conditions necessary to set Code P0122 exist.
2. Simulates Code P0123 (high voltage). If PCM recognizes high signal voltage and sets Code P0123, PCM and wiring are okay.
3. Simulates a high signal voltage. Checks signal circuit for an open.

Tech 1 displays throttle position in volts. Voltage should increase steadily as throttle is moved toward WOT. An open or short to ground in reference and signal circuits will cause Code P0122 to set. Check for

1. Poor Connection Or Damaged Harness: Inspect PCM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test: Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating the location of the malfunction.
3. TP Sensor Scaling: Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (.20-.74 volt) to greater than 4 volts when throttle is held at WOT position.

Code P0122 Flow Chart Throttle Position Sensor. Scheme 81

CODE P0123, THROTTLE POS SENS - 4.3L "S" & "T" SERIES P/U M/T

The Throttle Position (TP) sensor provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from .20-.74 volt or at idle to approximately 5 volts at Wide Open Throttle (WOT). TP sensor signal is one of the most important inputs used by PCM for fuel control and for most of the PCM control outputs. On non-adjustable TPS sensors, each time voltage drops below 1.25 volts and stops, the VCM assumes this value as 0 throttle angle and measures percent throttle from this point on.

On 4.3L, throttle position will default to 19 percent and Tech-1 will not indicate fault value. PCM will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. On 4.3L, if TP signal is greater than 1.25 volts, fault is present.
2. With TP sensor disconnected, TP sensor voltage should decrease and Code P0122 will set. This test verifies that PCM and wiring are okay.
3. Probing ground circuit with a test light check sensor ground circuit. A faulty sensor ground circuit will cause Code P0123 to set.

Tech 1 displays throttle position in volts. With closed throttle, ignition on, or at idle, voltage should be .20-.74 volt. If voltage is not as specified, replace TP sensor to set. Check for

1. Poor Connection Or Damaged Harness: Inspect PCM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test: Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating the location of the malfunction.
3. TP Sensor Scaling: Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (.20-.74 volt) to greater than 4 volts when throttle is held at WOT position.

Code P0123 Schematic (4.3L "S" & "T" Series Pickup M/T) Throttle Position Sensor. Scheme 82

Code P0123 Flow Chart Throttle Position Sensor. Scheme 83

Code P0123 Flow Chart. Scheme 84

CODE P0139, HEATED O2 SENS OPEN CKT - 4.3L "S" & "T" SERIES PICKUP M/T

The VCM applies a bias voltage of about 450 mV between Purple and Tan wire terminals. The heated oxygen sensor varies the voltage about 1000 mV if exhaust is rich, to about 100 mV if exhaust is lean. Sensor is like an open circuit and applies no voltage when below 600°F (315°C). An open sensor circuit or cold sensor causes open loop operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. This will determine open or closed loop operation.
2. This checks oxygen sensor heating element.
3. This will determine if sensor is at fault.
4. For this test only use a high impedance DVM. This checks continuity of circuits.

If oxygen sensor heater is not operating properly, system may go into open loop operation after extended idle. Tech-1 voltage varies between 0.1-1.0 volt in closed loop. Code P0139 sets in 1 minute if voltage is fixed between 0.35-0.55 volt. System will go into open loop in about 15 seconds.

Oxygen sample in sensor is needed for proper operation. Sample is directed through sensor wiring. All sensor wiring and connectors should be examined for breaks or contamination which may prevent reference oxygen from reaching sensor.

Code P0139 Schematic (4.3L "S" & "T" Series Pickup M/T) Heated O2 Sensor Open Circuit. Scheme 85

Code P0139 Flow Chart (4.3L "S" & "T" Series Pickup M/T) Heated O2 Sensor Open Circuit. Scheme 86

CODE P0171, LEAN EXHAUST - 4.3L "S" & "T" SERIES PICKUP M/T

The VCM applies a bias voltage of about 450 mV between Purple and Tan wire terminals. The heated oxygen sensor varies the voltage about 1000 mV if exhaust is rich, to about 100 mV if exhaust is lean.

Note. Test numbers refer to numbers on diagnostic chart.

1. Oxygen sensor voltage below 0.35 volt indicates fault is still present.

Using Tech-1, observe long term fuel trim values at different RPM and airflow conditions. If conditions for Code P0171 exist, long term fuel trim values will be around 150. Check for

1. Oxygen Sensor Wiring: Reference oxygen is supplied through sensor wiring. Check sensor wiring for breaks or contamination tha may prevent reference oxygen from reaching sensor.
2. Intermittent Ground: Check for an intermittent ground in wire between sensor and connector.
3. Fuel Contamination: Water near in-tank fuel pump can be delivered to injectors, causing a lean exhaust and setting Code P0171.
4. Fuel Pressure: If fuel pressure is too low, system will be lean. Monitor fuel pressure while driving at various speeds and loads to confirm.
5. Exhaust Leaks: If an exhaust leak exists, engine may pull outside air into exhaust and past sensor. Vacuum or crankcase leaks can cause a lean condition.
6. Sensor Ground Circuit: If Tan wire circuit is open, voltage at signal terminal may be about 0.45 volt. This may also cause a Code P0139 to be set.
7. Oxygen Sensor: If all checks are okay, heated oxygen sensor is faulty.

Code P0171 Flow Chart. Scheme 87

CODE P0172, RICH EXHAUST - 4.3L "S" & "T" SERIES PICKUP M/T

The VCM applies a bias voltage of about 4.5 volts between Purple and Tan wire terminals. The heated oxygen sensor varies the voltage about 1 volt if exhaust is rich, to about 0.1 volt if exhaust is lean. Code P0172 will set if oxygen sensor voltage remains high too long.

Note. Test numbers refer to numbers on diagnostic chart.

1. Oxygen sensor voltage above 0.69 volt indicates fault is still present.

Using Tech-1, observe long term fuel trim vaules at different RPM and airflow conditions. If conditions for Code P0171 exist, long term fuel trim values will be around 115. Check for

1. Oxygen Sensor Wiring: Reference oxygen is supplied through sensor wiring. Check sensor wiring for breaks or contamination that may prevent reference oxygen from reaching sensor.
2. Fuel Pressure: System will go rich is pressure is too high. VCM will compensate for some increase, but code will set if pressure gets too high.
3. Fuel Contamination: Check for oil contaminated fuel.
4. HEI Shielding: An open ground in ignition system reference low circuit may result in electrical interference. VCM looks at this at reference pulses. Pulses result in a higher than actual engine speed signal. VCM then delivers too much fuel, causing system to go rich. Tachometer will also show higher than actual engine speed, which can help in diagnosing problem.
5. MAP Sensor: If an output causes VCM to sense high manifold pressure (low vacuum), system will go rich. Disconnecting MAP sensor will allow VCM to set a fixed MAP value. Substitute a different MAP sensor if rich condition is gone when sensor is disconnected.
6. Pressure Regulator: Check for leaking fuel in pressure regulator diaphragm by checking for liquid fuel.
7. Throttle Position Sensor: An intermittent TP sensor output will cause system to go rich due to false indication of engine accelerating.
8. Engine Coolant Temperature Sensor: Check for shifted sensor that could cause a rich exhaust, but set Code P0117.

CODE P0320, IGN CNTRL ERROR - 4.3L "S/T" SERIES PICKUP - M/T

When system is running on ignition control module, when no voltage is on the bypass line, the DI module grounds the ignition control signal. The VCM expects to sense no voltage on the IC line during this condition. If it senses voltage it sets a Code P0320 and will not go into the IC mode.

When RPM for IC is reached and bypass voltage is applied, the IC will no longer be grounded in the DI module, so IC voltage will vary. VCM will disable the IC mode if short test fails or default to bypass mode if long test fails.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks to see if code is an intermittent.
2. Checks for a normal IC ground path through DI module. If IC Tan/Black wire is shorted to ground, it will also read less than 500 ohms. This will be checked later.
3. As test light voltage touches Tan/Black wire circuit, the module should switch, causing ohmmeter to over-range if meter is in 100-200 range. With meter in 10-20,000 range, position will be above 5000 ohms. The important thing is that the module switched.
4. If module did not switch, this step checks for IC circuit shorted to ground, bypass circuit open or faulty ignition module or connection.
5. This step confirms that code is set because of a faulty VCM and not an intermittent in circuitry.

Tech-1 tool does not have ability to help diagnose any Code P0320 problems. Check for intermittents. An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside insulation. Check for backed out connector terminals or broken down insulation.

Code P0320 Schematic (4.3L "S" & "T" Series Pickup M/T) Ignition Control Error. Scheme 88

Code P0320 Flow Chart (4.3L "S" & "T" Series Pickup M/T) Ignition Control Error. Scheme 89

CODE P0327, KNOCK SENSOR GROUNDED - 4.3L "S/T" PICKUP - M/T

Knock sensor circuit consists of 2 knock sensors connected in parallel and a single wire to VCM. Each knock sensor has a resistance value of about 8600 ohms. VCM supplies a 5 volt reference to sensors, but because of sensor resistance it is cut to about 2.5 volts. When a knock occurs, a small AC voltage if generated by the sensors on top of the existing 2.5 volts. An AC voltage monitor in VCM will detect this voltage and provide a signal to begin retarding spark until knock diminishes.

Check Dark Blue wire circuit for a potential ground. Check knock sensor(s) connections. A worn connector may not fit properly because of underhood temperatures. Check for proper PROM installation.

Code P0327 Schematic (4.3L "S" & "T" Series Pickup M/T) Knock Sensor Grounded. Scheme 90

Code P0327 Flow Chart (4.3L "S" & "T" Series Pickup M/T) Knock Sensor Grounded. Scheme 91

CODE P0328, KNOCK SENSOR OPEN - 4.3L "S/T" PICKUP - M/T

Knock sensor circuit consists of 2 knock sensors connected in parallel and a single wire to VCM. Each knock sensor has a resistance value of about 8600 ohms. VCM supplies a 5 volt reference to sensors, but because of sensor resistance it is cut to about 2.5 volts. When a knock occurs, a small AC voltage if generated by the sensors on top of the existing 2.5 volts. An AC voltage monitor in VCM will detect this voltage and provide a signal to begin retarding spark until knock diminishes.

Note. Test numbers refer to numbers on diagnostic chart.

1. VCM applies 5 volts on sensor circuit which should be present at connectors when sensors are disconnected.
2. By verifying sensor resistance, problem can be narrowed to connector, sensor installation or VCM.
3. Improperly installed sensor can prevent sensor from grounding to block.

Check Dark Blue wire circuit for a potential open. Check knock sensor(s) connections. A worn connector may not fit properly because of underhood temperatures. Check for proper PROM installation.

Code P0328 Flow Chart (4.3L "S" & "T" Series Pickup M/T) Knock Sensor Open. Scheme 92

CODE P0400, (EGR) ERROR - 4.3L "S" & "T" SERIES PICKUP M/T

VCM operates an EVRV solenoid, that is pulse width modulated to control EGR valve. Solenoid is normally closed. By providing a ground path, VCM energizes the solenoid which allows vacuum to pass to EGR valve. VCM can vary solenoid on time from zero to 100% depending on load conditions.

The VCM monitors EGR effectiveness by de-energizing EGR control solenoid, thereby shutting off vacuum to EGR diaphragm. With EGR valve closed and oxygen sensor operating normally, short fuel trim count will be greater than during normal EGR operation. If test passed, it will be run only once during ignition cycle. If test failed, it must fail twice before Code P0400 will be set.

Note. Test numbers refer to numbers on diagnostic chart.

1. With ignition on and engine stopped, solenoid should not be energized and vacuum should not pass to EGR valve. Grounding diagnostic terminal will energize solenoid and allow vacuum to pass.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
3. Vehicle must be driven during this test to ensure sufficient load to operate EGR.

Before replacing VCM, check resistance of each VCM controlled relay and solenoid coil, except EVRV. EVRV is solid state. Replace any solenoid when resistance measures less than 2 ohms. An intermittent may be caused by a broken wire inside insulation. Check for

1. Poor Connection Or Damaged Harness: Inspect PCM harness connectors for backed out EGR control circuit terminal, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Code P0400 Schematic (4.3L "S" & "T" Series Pickup M/T) (EGR) Error. Scheme 93

Code P0400 Flow Chart (4.3L "S" & "T" Series Pickup M/T) (EGR) Error. Scheme 94

CODE P0500, VSS CIRCUIT - 4.3L "S" & "T" SERIES PICKUP M/T

The Vehicle Speed Sensor (VSS) is magnetic induction type. Gear teeth pressed on outside of output shaft induce an AC current in sensor as shaft rotates. Signal goes directly to VCM. This pulsing action happens about 2000 times per mile and VCM will calculate vehicle speed based on time between pulses.

Note. Test numbers refer to numbers on diagnostic chart.

1. This determines if code is set as result of a hard failure or an intermittent.
2. This checks wiring, connections and VCM and VSS.

Check that proper calibration is in the VCM for vehicle speedometer.

Code P0500 Schematic (4.3L "S" & "T" Series Pickup M/T) VSS Circuit. Scheme 95

Code P0500 Flow Chart (4.3L "S" & "T" Series Pickup M/T) VSS Circuit. Scheme 96

CODE P1222, FUEL PUMP VLTGE LOW - 4.3L "S/T" PICKUP - M/T

The status of the fuel pump circuit is monitored by the VCM and is used to compensate fuel delivery based on system voltage. Signal is also used to store a Code P1222 if fuel pump relay is defective or fuel pump voltage is lost while engine is running. There should be about 12 volts on fuel pump signal circuit for at least 2 seconds after ignition is turned on, or when reference pulses are being received by VCM.

Fuel pump should run for 2 seconds and then turn off if VCM senses no reference pulses. Fuel pump relay is located in convenience center.

Code P1222 Schematic (4.3L "S" & "T" Series Pickup M/T) Fuel Pump Voltage Low. Scheme 97

SUMMARY

If hard fault codes are not present and driveability symptoms or intermittent codes exist, proceed to appropriate TESTS W/O CODES article in this section below for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.), or intermittent diagnostic procedures.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

See also:
• READING TROUBLE CODES
• RETRIEVING CODES (NON-SCAN)
• HARD OR INTERMITTENT TROUBLE CODE DETERMINATION
• SCAN TESTER DATA
• SCAN TESTER USAGE
• CLEARING TROUBLE CODES
• SCAN TESTER DATA