Engine Controls - Tests W/codes - V6: Diagnosis GMC Cab & Chassis C3500

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.

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»(ref-10386) 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»(ref-10391-S02522944022001010200000) . For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, proceed to INTERMITTENTS in the appropriate «TESTS W/O CODES»(ref-10392) 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»(ref-10392) article. From there you will be sent to the appropriate «SYSTEM/COMPONENT TESTS»(ref-10387) article.
4. After repairs are made, clear trouble codes and perform FIELD SERVICE MODE check in «BASIC TESTING»(ref-10386) article.

Scheme 73

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»(ref-10387) article. Turn ignition on. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of ALDL connector. (Scheme 73) 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»(ref-10386) 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»(ref-10386) 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»(ref-10391-S19392607072001010200000) .
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.

Diagnostic Aids

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

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.

TROUBLE CODE CHARTS

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

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 74

Code 13 Flow Chart (Except 4.3L Turbo) Open Oxygen Sensor Circuit. Scheme 75

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 76

Code 13 Flow Chart (4.3L Turbo) Open Oxygen Sensor Circuit. Scheme 77

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 78

Code 14 Flow Chart Coolant Sensor Signal Voltage Low. Scheme 79

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 80

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 81

Code 21 Schematic TPS Signal Voltage High. Scheme 82

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 83

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 84

Code 23 Flow Chart (2.5L & 4.3L Turbo) MAT Sensor Temperature Low. Scheme 85

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 86

Code 24 Schematic (Astro, Safari & G, P, R & V W/O 4L80-E Transmission) Vehicle Speed Sensor. Scheme 87

Code 24 Flow Chart (Except 3.1L, C & K Series & Models W/O 4L80-E Transmission) Vehicle Speed Sensor. Scheme 88

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 89

Code 24 Flow Chart (C & K Series W/O 4L80-E Trans.) Vehicle Speed Sensor. Scheme 90

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 91

Code 24 Flow Chart (C & K Series 2WD W/4L80-E Trans.) Vehicle Speed Sensor. Scheme 92

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 93

Code 24 Flow Chart (2WD W/4L80-E Trans. Except C & K Series) Vehicle Speed Sensor. Scheme 94

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 95

Code 24 Flow Chart (4WD W/4L80-E Trans.) Vehicle Speed Sensor. Scheme 96

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 97

Code 24 Flow Chart (3.1L) Vehicle Speed Sensor. Scheme 98

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 99

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 100

Code 32 Flow Chart (4.3L C, G, K & P Series W/4L80-E Trans.) EGR System Error. Scheme 101

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 102

Code 32 Flow Chart (All Engines W/4L80-E Trans. Except 4.3L) EGR System Error. Scheme 103

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 104

Code 32 Schematic (2.8L, 4.3L S & T Series & 7.4L) EGR System Error. Scheme 105

Code 32 Flow Chart (All W/O 4L80-E Trans. Except 3.1L & 4.3L Turbo) EGR System Error. Scheme 106

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 107

Code 32 Flow Chart (4.3L) EGR System Error. Scheme 108

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 109

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 110

Code 33 Flow Chart Map Sensor Signal Voltage High. Scheme 111

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 112

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.

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 113

Code 42 Schematic (All W/ 4L80-E Transmission) Electronic Spark Timing. Scheme 114

Code 42 Schematic (3.1L) Electronic Spark Timing. Scheme 115

Code 42 Schematic (All W/O 4L80-E Trans. Except 2.5L, 3.1L & 4.3L Turbo) Electronic Spark Timing. Scheme 116

Code 42 Schematic (4.3L Turbo) Electronic Spark Timing. Scheme 117

Code 42 Flow Chart Electronic Spark Timing. Scheme 118

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 119

Code 43 Flow Chart (Dual Sensors) Knock Sensor W/O Spark Module. Scheme 120

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 121

Code 43 Schematic (All Except 3.1L, 4.3L Turbo & Models W/4L80-E Transmission) Electronic Spark Control. Scheme 122

Code 43 Flow Chart (All Except 4.3L Turbo & Models W/4L80-E Transmission) Electronic Spark Control. Scheme 123

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.

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.

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

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

See also:
• WIRING DIAGRAMS