Engine Controls - Tests W/codes - v6 GMC Typhoon I

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 article BASIC TESTING . Failure to do so may result in lost diagnostic time.

If no faults were found while in the BASIC TESTING article proceed with DIAGNOSTIC PROCEDURE. If no fault codes or only a non-running Code 12 is present and driveability problems exist, proceed to 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 TESTS W/O CODES article.

SELF-DIAGNOSTIC SYSTEM

All vehicle are equipped with either an Electronic Control Module (ECM) or Powertrain Control Module (PCM). Unless specifically stated, references to ECM also apply to PCM 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. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in ECM memory. To retrieve stored codes, see READING TROUBLE CODES . Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.

Hard Failures

Hard failures cause SERVICE ENGINE SOON light 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 SERVICE ENGINE SOON light 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 TESTS W/O CODES article.

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 «BASIC TESTING»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-basic-testing) article.
2. If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes cause SERVICE ENGINE SOON light to illuminate continuously with engine running. See «HARD OR INTERMITTENT TROUBLE CODE DETERMINATION»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-tests-wcodes-v6__hard-or-intermittent-trouble-code-determination) . For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, proceed to INTERMITTENTS in the appropriate «TESTS W/O CODES»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-tests-wo-codes) article in the section. Exceptions are Code 13, 15, 24, 44 and 45 charts, which can help diagnose intermittent codes.
3. If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in «TESTS W/O CODES»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-tests-wo-codes) article. From there you will be sent to the appropriate «SYSTEM/COMPONENT TESTS»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) article.
4. After repairs are made, clear trouble codes and perform FIELD SERVICE MODE check in «BASIC TESTING»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-basic-testing) article.

Scheme 99

1. Turn ignition on with engine off. SERVICE ENGINE SOON light should glow. Locate Assembly Line Data Link (ALDL) connector, attached to ECM wiring harness. Most ALDL connectors are located under dash on driver's side of vehicle. For exact location of ALDL, see COMPONENT LOCATIONS illustration in «SYSTEM/COMPONENT TESTS»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) article. Turn ignition on. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of ALDL connector. (Scheme 99) NOTE: Inserting jumper wire into test and ground terminals of ALDL connector with engine running will cause fuel-injected vehicles to enter field service mode and codes will not flash. See FIELD SERVICE MODE in «BASIC TESTING»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-basic-testing) article.
2. SERVICE ENGINE SOON light should flash codes. Each code is flashed 3 times. If codes DO NOT flash, perform DIAGNOSTIC CIRCUIT CHECK in «BASIC TESTING»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-basic-testing) article. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

READING TROUBLE CODES

Note. Trouble codes retrieved from ECM/PCM may be related to either engine or transmission. For engine related codes, use this article. For transmission related codes, see the TESTS W/CODES - DIESEL (for Diesel engines) or and TESTS W/CODES - TRANSMISSION (if transmission codes are displayed) article. To identify which codes relate to transmission or engine, see TROUBLE CODE IDENTIFICATION TABLE below.

The ECM stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Read trouble codes by counting SERVICE ENGINE SOON light flashes or with diagnostic Scan tester connected to the ALDL connector. 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, SERVICE ENGINE SOON light flashes can be read by grounding ALDL diagnostic 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 ALDL 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 IDENTIFICATION

Note. Trouble code charts should only be used if SERVICE ENGINE SOON light 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 Codes 51, 52 or 55 are displayed with another code, start with 50-series code first and proceed to low profile 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»(/gmc/typhoon/i-1991-1993/remont/testing-diagnostics/#engine-controls-tests-wcodes-v6__clearing-trouble-codes) .
2. Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. SERVICE ENGINE SOON light should go out. Run warm engine at specified curb idle for 2 minutes and note SERVICE ENGINE SOON light.
3. If SERVICE ENGINE SOON light 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 SERVICE ENGINE SOON light 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 ALDL connector. Turn ignition switch to OFF position and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead.

ECM LOCATION

For ECM locations, see appropriate COMPONENT LOCATIONS illustration in SYSTEM/COMPONENT TESTS article.

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

SERVICE ENGINE SOON light indicates operational mode of engine if ALDL 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 BASIC TESTING article.

SPECIAL TOOLS (DIAGNOSTIC)

Note. A special Scan tester, plugged into the ALDL, 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 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 BASIC TESTING article. If vehicle does not pass diagnostic circuit check, information received by Scan tester may be invalid.

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 back-probe 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 ALDL 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 ALDL test terminal. If same codes are not flashed by SERVICE ENGINE SOON light as were indicated by Scan tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.

SCAN 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). Data parameters are updated every 1 1/2 seconds. Not all devices and systems are used on all models. For additional information, see tester owner's manual.

SCAN TESTER DATA

SUMMARY

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

TROUBLE CODE CHARTS

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

Except 4.3L Turbo

When exhaust temperature is less than 600°F (316°C), the O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not all system to entire closed loop.

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 1 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 ground circuit No. 413. An open circuit at sensor signal terminal or ground terminal will result in a Code 13.

CODE 13 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 13 ECM TERMINAL & CIRCUIT WIRING ID (ALL OTHERS)

Code 13 Schematic (Except 4.3L Turbo) Open Oxygen Sensor Circuit. Scheme 100

Code 13 Flow Chart (Except 4.3L Turbo) Open Oxygen Sensor Circuit. Scheme 101

4.3L Turbo

When exhaust temperature is less than 600°F (316°C), the O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not all system to entire closed loop.

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, O2 signal voltage is steady between .35-.55 volt and throttle position sensor signal is above idle. All conditions must be met for at least 1 minute.
2. This checks the oxygen sensor heating element. Resistance should be 3.5-14 ohms at 662°F (350°C).
3. This determines if fault is in O2 sensor, ECM or wiring.
4. 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.
5. If the A/C fuse is blown, check the A/C control circuit or generator for short circuits.

Verify a clean, tight connection for ground circuit No. 413. An open circuit at sensor signal terminal or ground terminal will result in a Code 13. Both oxygen sensor and heating element must be functioning properly to enable closed loop operation.

Code 13 Schematic (4.3L Turbo) Open Oxygen Sensor Circuit. Scheme 102

Code 13 Flow Chart (4.3L Turbo) Open Oxygen Sensor Circuit. Scheme 103

CODE 14, COOLANT SENSOR SIGNAL VOLTAGE LOW

The ECM uses coolant temperature sensor inputs in determining control of fuel delivery, engine timing (EST), idle (IAC) and converter clutch (TCC). As the engine warms, the sensor resistance reduces. At normal operating temperature, voltage signal will be about .5-1.1 volts (4.3L Turbo) or 1.5-2.0 volts (all other models) at ECM 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 3 seconds on 4.3L Turbo or 6 seconds on all other models.
2. This simulates conditions for a Code 15. If the ECM recognizes the open circuit by displaying a low temperature, the ECM and wiring are not at fault.

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

CODE 14 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

CODE 14 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 14 ECM TERMINAL & CIRCUIT WIRING ID (2.8L, 3.1L, 4.3L EXCEPT TURBO, 5.0L & 5.7L C & K SERIES)

CODE 14 ECM TERMINAL & CIRCUIT WIRING ID (ALL OTHERS)

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

Code 14 Schematic Coolant Sensor Signal Voltage Low. Scheme 104

Code 14 Flow Chart Coolant Sensor Signal Voltage Low. Scheme 105

CODE 15, COOLANT SENSOR SIGNAL VOLTAGE HIGH

As the engine warms, the sensor resistance reduces and voltage drops. At normal operating temperature, voltage signal will be about .5-1.1 volts (4.3L Turbo) or 1.5-2.0 volts (all other models) at ECM coolant sensor signal terminal. If sensor signal circuit opens, ECM will see -40°F (-40°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 the ECM recognizes the grounded circuit and displays a high temperature, the 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 the engine is allowed to cool overnight, the 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 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

CODE 15 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 15 ECM TERMINAL & CIRCUIT WIRING ID (2.8L, 3.1L, 4.3L EXCEPT TURBO, 5.0L & 5.7L C & K SERIES)

CODE 15 ECM TERMINAL & CIRCUIT WIRING ID (ALL OTHERS)

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

Code 15 Flow Chart Coolant Sensor Signal Voltage High. Scheme 106

CODE 21, TPS SIGNAL VOLTAGE HIGH

The Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from about .50 volt at idle to 4.5 volts at wide open throttle. On models with non-adjustable TPS, 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. On 2.8L, Code 21 may set if MAP sensor signal less than 2 volts.
2. This test simulates conditions for Code 22. If the ECM recognizes the low voltage signal and sets Code 22, the 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 1.0 volt (4.3L Turbo) or less than 1.25 volts (all other models). TPS voltage should increase at a steady rate to 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 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

CODE 21 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 21 ECM TERMINAL & CIRCUIT WIRING ID (2.8L, 3.1L, 4.3L EXCEPT TURBO, 5.0L & 5.7L C & K SERIES)

CODE 21 ECM TERMINAL & CIRCUIT WIRING ID (ALL OTHERS)

Code 21 Schematic TPS Signal Voltage High. Scheme 107

Code 21 Schematic TPS Signal Voltage High. Scheme 108

CODE 22, TPS SIGNAL VOLTAGE LOW

The 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 4.5 volts at wide open throttle.

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

1. This test confirms Code 22 and tests if the fault is a hard failure or an intermittent condition. Code 22 will set if engine is running, TPS voltage is less than .2 volt for 2-4 seconds.
2. This simulates Code 21. If the ECM recognizes a high voltage signal and sets Code 21, the ECM and wiring are not at fault.
3. On 2.8L, check and adjust TPS. On all others, replace TPS.
4. 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 (4.3L Turbo) or less than 1.25 volts (all other models). TPS voltage should increase at a steady rate to 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.

CODE 22 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

CODE 22 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 22 ECM TERMINAL & CIRCUIT WIRING ID (2.8L, 3.1L, 4.3L EXCEPT TURBO, 5.0L & 5.7L C & K SERIES)

CODE 22 ECM TERMINAL & CIRCUIT WIRING ID (ALL OTHERS)

Code 22 Flow Chart TPS Signal Voltage Low. Scheme 109

2.5L & 4.3L Turbo

The ECM supplies and monitors a voltage signal (4-6 volts) to the sensor. When temperatures are low, sensor resistance is high and the ECM will see a high-monitored voltage signal. As temperature increases, sensor resistance decreases and voltage sensed by the 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 -22°F (-30°C) for 12 seconds and speed sensor signal is not present.
2. This simulates conditions for a Code 25. If the Scan tester displays a high temperature, the ECM and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits.

If the engine is allowed to cool overnight, the 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 ECM TERMINAL & CIRCUIT WIRING ID (2.5L)

CODE 23 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

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

Code 23 Schematic (2.5L & 4.3L Turbo) MAT Sensor Temperature Low. Scheme 110

Code 23 Flow Chart (2.5L & 4.3L Turbo) MAT Sensor Temperature Low. Scheme 111

Except 3.1L, C & K Series & Models W/4L80-E Transmission

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the Digital Ratio Adapter Controller (DRAC) which alternately grounds this circuit when it is receiving voltage pulses from the Vehicle Speed Sensor (VSS). Scan tester reading should closely match speedometer reading.

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

1. Code 24 will set if voltage on circuit No. 437 is constant, engine speed is greater than 1200 RPM, vehicle speed is less than 2 MPH vehicle is not in Park or Neutral and all conditions have been met for at least 5 seconds. These conditions are met during a road load deceleration.
2. This determines if DRAC is receiving the AC signal from the 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 aver age 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 the appropriate TESTS W/O CODES article.

Code 24 Schematic (4.3L Turbo) Vehicle Speed Sensor. Scheme 112

Code 24 Schematic (Astro, Safari & G, P, R & V W/O 4L80-E Transmission) Vehicle Speed Sensor. Scheme 113

Code 24 Flow Chart (Except 3.1L, C & K Series & Models W/O 4L80-E Transmission) Vehicle Speed Sensor. Scheme 114

C & K Series Except With 4L80-E Transmission

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the Digital Ratio Adapter Controller (DRAC) which alternately grounds this circuit when it is receiving voltage pulses from the Vehicle Speed Sensor (VSS). Scan tester reading should closely match speedometer reading.

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

1. Code 24 will set if voltage on circuit No. 437 is constant, engine speed is greater than 1200 RPM, vehicle speed is less than 2 MPH vehicle is not in Park or Neutral and all conditions have been met for at least 10 seconds. These conditions are met during a road load deceleration.
2. 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 aver age 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 TESTS W/O CODES article.

Code 24 Schematic (C & K Series W/O 4L80-E Trans.) Vehicle Speed Sensor. Scheme 115

Code 24 Flow Chart (C & K Series W/O 4L80-E Trans.) Vehicle Speed Sensor. Scheme 116

C & K Series 2WD With 4L80-E Transmission

VSS output sensor circuit consists of a magnetic induction type speed sensor, Digital Ratio Adapter Controller (DRAC), and wiring. Gear teeth cut in the outside diameter of the rear internal gear induce an alternating current in the sensor when drive wheels are turning. Signal is transmitted to the instrument cluster DRAC where it is converted to a square wave signal which is passed on to the PCM, cruise control, speedometer, and anti-lock brake system. Code 24 will set if gear selector is not in Park or Neutral, input speed is at least 3000 RPM and output speed is less than 200 RPM for at least one second. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

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

1. Test verifies VSS voltage at PCM.
2. Test verifies operation of VSS.
3. Test checks circuits No. 821 and 822 up to the instrument cluster.

Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in the TESTS W/O CODES article. While Code 24 is set, Scan tester will display an RPM derived from input speed.

Code 24 Schematic (C & K Series 2WD W/4L80-E Trans.) Vehicle Speed Sensor. Scheme 117

Code 24 Flow Chart (C & K Series 2WD W/4L80-E Trans.) Vehicle Speed Sensor. Scheme 118

2WD With 4L80-E Trans. Except C & K Series

VSS output sensor circuit consists of a magnetic induction type speed sensor, Digital Ratio Adapter Controller (DRAC), and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor when drive wheels are turning. Signal is transmitted to the instrument cluster DRAC where it is converted to a square wave signal which is passed on to the PCM, cruise control, speedometer, and anti-lock brake system. 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 200 RPM for at least 1.5 seconds. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

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

1. Test verifies VSS voltage at PCM.
2. Test verifies VSS signal to DRAC.
3. Test verifies VSS signal at sensor.

Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in the TESTS W/O CODES article. While Code 24 is set, Scan tester will display an RPM derived from input speed.

Code 24 Schematic (2WD W/4L80-E Trans. Except C & K Series) Vehicle Speed Sensor. Scheme 119

Code 24 Flow Chart (2WD W/4L80-E Trans. Except C & K Series) Vehicle Speed Sensor. Scheme 120

4WD With 4L80-E Transmission

VSS output sensor is a magnetic induction type. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor when drive wheels are turning. Since vehicle speed is taken from the 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 200 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 verifies VSS signal at sensor.

Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in the TESTS W/O CODES article. 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.

Code 24 Schematic (4WD W/4L80-E Trans.) Vehicle Speed Sensor. Scheme 121

Code 24 Flow Chart (4WD W/4L80-E Trans.) Vehicle Speed Sensor. Scheme 122

3.1L Engine

ECM applies and monitors a 12-volt signal on circuit No. 437 to Vehicle Speed Sensor (VSS). VSS is connected to speed sensor buffer which alternately grounds and opens circuit No. 437 when wheels are turning. This pulsing action takes place about 2000 times per mile. The voltage level and pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH. The ECM uses VSS information in calculations to determine vehicle adjustments. Scan tester reading should closely match speedometer reading when wheels are turning.

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

1. A Code 24 sets when MPH is less than 2 MPH, transmission is not in Park or Neutral, engine speed is greater than 1400 RPM, TPS is greater than 5 percent, circuit No. 437 voltage is constant and all of these conditions are met for 30 seconds. These conditions are met during a road load deceleration.
2. A steady 8-12 volts at ECM connector indicates VSS circuit is open or speed sensor is faulty. A voltage of less than one volt at IP connector indicates circuit No. 437 wire is shorted to ground. Disconnect vehicle speed sensor connector. If voltage is now greater than 10 volts, vehicle speed sensor buffer is faulty. If voltage remains less than 8 volts, circuit is grounded. If circuit is not grounded, check for faulty ECM connector or ECM. Before replacing the ECM, PROM should be checked for correct application.

A faulty or misadjusted Park/Neutral switch may set a false Code 24. Use Scan tester and check for proper signal in Drive, while wiggling shifter. Scan tester MPH reading should closely match speedometer when vehicle is moving. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.

Code 24 Schematic (3.1L) Vehicle Speed Sensor. Scheme 123

Code 24 Flow Chart (3.1L) Vehicle Speed Sensor. Scheme 124

The ECM applies and monitors a voltage signal (4-6 volts) to MAT sensor. When manifold air is cold, the sensor resistance is high and ECM sees a high signal voltage. As air warms, resistance decreases and voltage sensed by the 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 the 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 302°F (150°C) for 2.5L or 275°F (135°C) for 4.3L Turbo.

If the engine is allowed to cool overnight, the 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 ECM TERMINAL & CIRCUIT WIRING ID (2.5L)

CODE 25 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

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

Code 25 Flow Chart (2.5L & 4.3L Turbo) MAT Sensor Temperature High. Scheme 125

CODE 31, TURBO OVERBOOST (4.3L TURBO)

On turbocharged engines, exhaust gases pass from exhaust manifold through turbocharger, turning turbine blades. Compressor side of turbocharger also turns, pulling air through air filter and pushing it into intake manifold under pressure.

Wastegate is normally closed, but will open to by-pass exhaust gases when an overboost condition exists. This is determined through monitored MAP sensor signals. Under normal driving conditions, ECM energizes the wastegate control (solenoid) valve. This blocks off intake manifold pressure from wastegate actuator diaphragm. When overboost condition exists, ECM will de-energize the wastegate control (solenoid) valve, allowing intake manifold pressure to act upon actuator diaphragm, opening wastegate.

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

1. Code 31 will set when Code 33 is not set and excessive manifold boost pressure is sensed for 2 seconds. This step tests the ability of the actuator to open wastegate. Code 31 may be caused by circuit No. 471 shorted to ground, a sticking wastegate or actuator, control (solenoid) valve stuck in the closed position, a cut or pinched hose, or a faulty ECM.
2. After pressure is applied to valve and then removed, actuator should slowly close wastegate. If not, vent in control (solenoid) valve could be plugged.
3. With ignition on and ALDL test terminal "B" grounded, control (solenoid) valve should be energized. This closes off the manifold to wastegate actuator.
4. This checks the electrical control portion of the system. With ignition on and engine not running, solenoid should not be energized.

Code 31 Schematic (4.3L Turbo) Turbo Overboost. Scheme 126

Code 31 Flow Chart (4.3L Turbo) Turbo Overboost. Scheme 127

4.3L C, G, K, & P Series W/4L80-E Transmission

PCM controls a solenoid regulating vacuum to the 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.

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

1. When test terminal "B" of the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the EGR valve. Vacuum should hold.
2. When jumper wire is removed from terminal "B", vacuum to the EGR valve should bleed through a vent in the 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 the 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 (4.3L C, G, K & P Series W/4L80-E Trans.) EGR System Error. Scheme 128

Code 32 Flow Chart (4.3L C, G, K & P Series W/4L80-E Trans.) EGR System Error. Scheme 129

All Engines With 4L80-E Trans. Except 4.3L

PCM controls a solenoid that regulates vacuum to the 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 indicate cruise condition and throttle position is 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 test terminal "B" of the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the 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.

Code 32 Schematic (All Engines W/4L80-E Trans. Except 4.3L) EGR System Error. Scheme 130

Code 32 Flow Chart (All Engines W/4L80-E Trans. Except 4.3L) EGR System Error. Scheme 131

All W/O 4L80-E Trans Except 3.1L, & 4.3L Turbo

ECM controls a solenoid that regulates vacuum to the EGR valve. The 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.

ECM checks EGR operation when vehicle speed is greater than 50 MPH, engine vacuum indicates cruise and throttle is steady.

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 test terminal "B" of the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the EGR valve. Vacuum should hold.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
3. SERVICE ENGINE SOON light should also flash while diagnostic terminal is grounded. If it does not this may indicate the Quad-Driver has been damaged by low TCC circuit resistance.
4. 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.

Code 32 Schematic (2.5L, 4.3L Non-Turbo & S & T 5.0L & 5.7L) EGR System Error. Scheme 132

Code 32 Schematic (2.8L, 4.3L S & T Series & 7.4L) EGR System Error. Scheme 133

Code 32 Flow Chart (All W/O 4L80-E Trans. Except 3.1L & 4.3L Turbo) EGR System Error. Scheme 134

ECM operates a Electronic Vacuum Regulator Valve (EVRV) to control vacuum to the EGR valve. Regulator normally allows only a small amount of vacuum to pass to EGR valve. When ECM grounds EVRV drive circuit, EVRV opens, allowing enough vacuum to pass through to open EGR valve. ECM monitors EGR effectiveness by periodically checking fuel integrator counts. This is done when vehicle speed is greater than 50 MPH, MAP sensor signal indicates cruise condition, throttle position is constant and all conditions have been met twice for 60 seconds.

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

1. With ignition on and engine off, EVRV should not be energized or allow enough vacuum through to open EGR valve. When test terminal "B" of the ALDL connector is grounded, EVRV will energize, allowing vacuum to the EGR valve.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.

Code 32 Schematic (3.1L) EGR System Error. Scheme 135

Code 32 Flow Chart (3.1L) EGR System Error. Scheme 136

ECM controls a solenoid that regulates vacuum to the EGR valve. The 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.

Diagnostic chart covers checks for the entire EGR system. If no trouble is found while performing chart routine, fault is an intermittent electrical problem or a sticky EGR valve.

Vacuum switch in the EGR solenoid is not used. If EGR valve sticks open, a rough idle will result. If EGR solenoid vent filter becomes plugged, EGR valve will remain open or close slowly. An inoperative check valve in the ported vacuum supply line will result in faulty EGR system operation.

Code 32 Schematic (4.3L) EGR System Error. Scheme 137

Code 32 Flow Chart (4.3L) EGR System Error. Scheme 138

CODE 32, EGR SYSTEM ERROR - 4.3L 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 circuits No. 435 and 439 with a test light and grounding ALDL 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.

Note. For circuit schematic, see WIRING DIAGRAMS article.

CODE 32, Flowchart, EGR System Error (4.3L W/Linear EGR). Scheme 139

CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH

The 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 the 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 the ECM recognizes and indicates low MAP signal, the ECM and 5-volt reference and MAP signal circuits are not at fault.

With the ignition switch in the "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 the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. 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 ECM TERMINAL & CIRCUIT WIRING ID (ASTRO & SAFARI 4.3L & 2.5L S SERIES)

CODE 33 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 33 ECM TERMINAL & CIRCUIT WIRING ID (2.8L, 3.1L & V8 C & K SERIES)

CODE 33 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

CODE 33 ECM TERMINAL & CIRCUIT WIRING ID (4.3L NON-TURBO & V8 EXCEPT C & K SERIES)

Code 33 Schematic Map Sensor Signal Voltage High. Scheme 140

Code 33 Flow Chart Map Sensor Signal Voltage High. Scheme 141

CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW

The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If the MAP sensor fails, the 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 terminals "B" to "C" 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 the ECM recognizes the voltage as greater than 4 volts (high MAP voltage signal), indicating the ECM and MAP signal circuit are not at fault.

With the ignition switch in the ON position and engine off, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparing BARO readings with a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. 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 stored Code 22 is stored, check MAP signal circuit for short to ground.

CODE 34 ECM TERMINAL & CIRCUIT WIRING ID (ASTRO & SAFARI 4.3L & 2.5L S SERIES)

CODE 34 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 34 ECM TERMINAL & CIRCUIT WIRING ID (2.8L, 3.1L & V8 C & K SERIES)

CODE 34 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

CODE 34 ECM TERMINAL & CIRCUIT WIRING ID (4.3L NON-TURBO & V8 EXCEPT C & K SERIES)

Code 34 Flow Chart Map Sensor Signal Voltage Low. Scheme 142

CODE 35, IDLE SPEED ERROR 2.5L & 4.3L TURBO

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 the extend position before engine speed reduces. This is normal on some engines. Fully extending the 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 the node light should flash Red and Green, while the IAC valve is cycled. While the 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, the idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary and disconnecting the IAC may not help. Scan 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 the 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 the 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 ECM TERMINAL & CIRCUIT WIRING ID (2.5L)

CODE 35 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

Code 35 Schematic (2.5L & 4.3L Turbo) Idle Speed Error. Scheme 143

Code 35 Flow Chart (2.5L & 4.3L Turbo) Idle Speed Error. Scheme 144

CODE 42, ELECTRONIC SPARK TIMING (EST)

Code 42 indicates ECM has seen an open or short to ground in HEI EST 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 EST ground path through the ignition module. If circuit No. 423 is shorted to ground, reading will be less than 500 ohms.
3. As test light voltage touches circuit No. 424, the 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 the module switched.
4. If module did not switch, this step tests for a short in circuit No. 423, an open in circuit No. 424 and a faulty ignition module connection or module.
5. This step confirms Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.

The Scan tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in TESTS W/O CODES article.

Code 42 Schematic (2.5L) Electronic Spark Timing. Scheme 145

Code 42 Schematic (All W/ 4L80-E Transmission) Electronic Spark Timing. Scheme 146

Code 42 Schematic (3.1L) Electronic Spark Timing. Scheme 147

Code 42 Schematic (All W/O 4L80-E Trans. Except 2.5L, 3.1L & 4.3L Turbo) Electronic Spark Timing. Scheme 148

Code 42 Schematic (4.3L Turbo) Electronic Spark Timing. Scheme 149

Code 42 Flow Chart Electronic Spark Timing. Scheme 150

Knock Sensor W/O Spark Module (Dual Sensors) - "L", "M", "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 (Dual Sensors) Knock Sensor W/O Spark Module. Scheme 151

Code 43 Flow Chart (Dual Sensors) Knock Sensor W/O Spark Module. Scheme 152

All Except 4.3L Turbo & Models With 4L80-E Transmission

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

1. If conditions for a Code 34 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 the 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 the signal voltage on the spark retard line remains low, this test should cause the signal on that line to go high. The 12-volt signal should be seen by the ECM as a no knock signal if the ECM and wiring are okay.
4. This test determines if the knock signal is detected on the sensor-to-controller line or if the ESC module is at fault.
5. If sensor line is routed too close to secondary ignition wires, the ESC module may see the interference as a knock signal.
6. This checks ground circuit to module. An open ground will cause the voltage on the monitored line to remain constant at about 12 volts. This would cause the Code 43 functional test to fail.
7. This should generate a knock signal to the controller. This determines if the ESC controller is operating correctly.

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

Code 43 Schematic (3.1L) Electronic Spark Control. Scheme 153

Code 43 Schematic (All Except 3.1L, 4.3L Turbo & Models W/4L80-E Transmission) Electronic Spark Control. Scheme 154

Code 43 Flow Chart (All Except 4.3L Turbo & Models W/4L80-E Transmission) Electronic Spark Control. Scheme 155

4.3L Turbo & Models With 4L80-E Transmission

ECM/PCM applies and monitors a 5-volt DC signal to the 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 the DC signal to the ECM/PCM. Knock signal intensity is dependent upon knock signal level.

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 the current time.
2. This step determines the state of the 5-volt reference signal applied to the sensor.
3. Checks knock sensor internal resistance.

Code 43 Schematic (4.3L Turbo & Models W/4L80-E Trans.) Electronic Spark Control. Scheme 156

Code 43 Flow Chart (4.3L Turbo & Models W/4L80-E Trans.) Electronic Spark Control. Scheme 157

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 at ECM is less than .2 volt for 50 seconds or more and system is operating in "closed loop".

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

O2 Sensor Wire

O2 sensor wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.

Fuel Contamination

Water, even small amounts, near the 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 BASIC TESTING article.

Exhaust Leaks

If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.

If Code 44 is intermittent, see INTERMITTENTS in the article TESTS W/O CODES .

CODE 44 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 44 ECM TERMINAL & CIRCUIT WIRING ID (ALL OTHERS)

Code 44 Flow Chart (Except 4.3L Turbo) Lean Exhaust Indication. Scheme 158

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 at ECM is less than .2 volt for 50 seconds or more and system is operating in "closed loop".

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

O2 sensor wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.

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

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 BASIC TESTING article.

If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.

If Code 44 is intermittent, see INTERMITTENTS in the article TESTS W/O CODES .

Code 44 Flow Chart (4.3L Turbo) Lean Exhaust Indication. Scheme 159

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 .7 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 BASIC TESTING article. The ECM can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.

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 the 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) the system can go rich. Disconnecting the 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 the 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 the ECM as a rich mixture, causing the 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 TESTS W/O CODES article.

CODE 45 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 45 ECM TERMINAL & CIRCUIT WIRING ID (ALL OTHERS)

Code 45 Flow Chart (Except 4.3L Turbo) Rich Exhaust Indication. Scheme 160

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 .7 volt for 60 seconds or more.

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

If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see TESTS W/CODES - DIESEL (for Diesel engines) or TESTS W/CODES - V6 (for Gas engines) and the article TESTS W/CODES - TRANSMISSION (if transmission codes are displayed). The ECM can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.

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 the increased RPM signal. "Scan" tester will show higher than actual RPM, which can help diagnose problem.

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

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

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

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 the ECM as a rich mixture, causing the ECM to set Code 45.

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

Code 45 Flow Chart (4.3L Turbo) Rich Exhaust Indication. Scheme 161

CODE 51, FAULTY PROM/MEM-CAL

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 (EXCEPT 2.5L)

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 (2.5L)

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 54, FUEL PUMP CIRCUIT

Code 54 will set if the ECM does not see 12 volts on the fuel pump signal voltage monitor during the first 2 seconds after ignition is turned on.

Code 54 Schematic (Astro, Safari, S & T Series) Fuel Pump Circuit. Scheme 162

Code 54 Schematic (C, K, R, V, G, & P Series W/O 4L80-E Trans) Fuel Pump Circuit. Scheme 163

Code 54 Schematic (All Models W/4L80-E Trans. Except G Series) Fuel Pump Circuit. Scheme 164

Code 54 Schematic (G Series W/4L80-E Trans.) Fuel Pump Circuit. Scheme 165

Code 54 Schematic (3.1L) Fuel Pump Circuit. Scheme 166

Code 54 Flow Chart Fuel Pump Circuit. Scheme 167

CODE 55, ECM ERROR (EXCEPT 2.5L)

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

See also:
• BASIC TESTING
• TESTS W/O CODES
• SYSTEM/COMPONENT TESTS
• TESTS W/CODES - V6
• WIRING DIAGRAMS
• READING TROUBLE CODES
• HARD OR INTERMITTENT TROUBLE CODE DETERMINATION
• CLEARING TROUBLE CODES