Diagnostic Aids

If engine runs okay, check the following:

• Light bulb.
• Purple wire from light bulb to ECM for open.
• METER fuse.

If fuse is blown, this may result in no Oil or Generator light, Seat Belt Reminder light, etc. If engine cranks, but will not run, check for the following conditions:

• Battery fuse or fusible link open.
• ECM ignition fuse open.
• Battery Red/White wire to ECM open.
• Ignition Red/Green wire to ECM open.
• Poor connection to ECM.

Fig 1: Chart A-1, Flow Chart, Won't Flash Code 12/No Check Engine Light

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Fig 2: Chart A-1, Schematic, Won't Flash Code 12/No Check Engine Light

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Normal scan voltage varies between .1-1.0 volt (100-999 mV) while in closed Loop. Code 13 will set within one minute if voltage remains between .347-.547 volt, but system will go into open loop in about 15 seconds.

Verify ground connections are clean and tight at terminals D6 and D7. An open circuit in these terminals will result in Code 13. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 13, Flow Chart, O2 Sensor Ckt./Open Ckt. (VIN 5)

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Fig 2: Code 13, Schematic, O2 Sensor Ckt./Open Ckt. (VIN 5)

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Normal scan voltage varies between .1-1.0 volt (100-999 mV) while in closed loop. Code 13 will set within one minute if voltage remains between .347-.547 volt, but system will go into open loop in about 15 seconds.

Verify ground connections are clean and tight at terminals D6 and D7. An open circuit in these terminals will result in Code 13. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 13, Flow Chart, O2 Sensor Ckt./Open Ckt. (VIN 6)

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Fig 2: Code 13, Schematic, O2 Sensor Ckt./Open Ckt. (VIN 6)

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After engine is started, engine coolant temperature should rise to approximately 194°F (90°C) and stabilize when thermostat opens. Code 14 is set if Gray/Black wire is shorted to ground. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 14, Flow Chart, CTS Ckt./High Temp. Indicated

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Fig 2: Code 14, Schematic, CTS Ckt./High Temp. Indicated

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After engine is started, engine coolant temperature should rise to approximately 194°F (90°C) and stabilize when thermostat opens. If engine is allowed to cool to ambient temperature, or overnight, coolant and IAT sensor resistance or temperature may be checked with a Scan tester. Readings for both components should be the same, or close.

Code 14 is set if Gray/Black wire is open. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 14, Flow Chart, CTS Ckt./Low Temp. Indicated

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Fig 2: Code 14, Schematic, CTS Ckt./Low Temp. Indicated

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With throttle closed, TPS voltage should be less than 1.25 volt. TPS voltage should steadily increase as throttle is opened. Code 21 will appear if Yellow/Blue wire is shorted to voltage, or ground wire is open. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 21, Flow Chart, TPS Ckt./Signal Voltage High

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Fig 2: Code 21, Schematic, TPS Ckt./Signal Voltage High

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With throttle closed, TPS voltage should be less than 1.25 volts. TPS voltage should steadily increase as throttle is opened. Code 21 will appear if Blue/Orange or Yellow/Blue wire is open or shorted. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 21, Flow Chart, TPS Ckt./Signal Voltage Low

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Fig 2: Code 21, Schematic, TPS Ckt./Signal Voltage Low

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When engine is allowed to cool to ambient temperature, or overnight, CTS and IAT sensor resistance or temperature may be checked with a Scan tester. Readings of components should be close to each other.

Code 23 is set if Blue/Black wire or Black wire is open. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 23, Flow Chart, IAT Sensor Ckt./Low Temp. Indicated

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Fig 2: Code 23, Schematic, IAT Sensor Ckt./Low Temp. Indicated

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When engine is allowed to cool to ambient temperature, or overnight, CTS and IAT sensor resistance or temperature may be checked with a Scan tester. Readings of components should be close to each other.

Code 23 is set if Blue/Black wire is shorted to ground. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 23, Flow Chart, IAT Sensor Ckt./High Temp. Indicated

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Fig 2: Code 23, Schematic, IAT Sensor Ckt./High Temp. Indicated

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Scan tester should indicate a vehicle speed whenever the drive wheels are turning faster than 3 MPH.

Check White wire (A10) for proper connection. Ensure wire connector is clean and tight, and harness is routed correctly. On automatic transmission, a faulty or misadjusted Park/Neutral switch can result in a false Code 24. Use a Scan tester and check for proper signal while in Drive. To check P/N switch, refer to I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 24, Flow Chart, Vehicle Speed Sensor Ckt.

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Fig 2: Code 24, Schematic, Vehicle Speed Sensor Ckt.

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A faulty connection or "shifted" EGR gas temperature sensor could cause Code 32 to set. To check for a "shifted" EGR gas temperature sensor, connect a DVOM to Gray/Black and Black wires. Start engine and check resistance of EGR gas temperature sensor against values in EGR TEMPERATURE SENSOR RESISTANCE table. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 32, Flow Chart, EGR System Error (Calif. Only)

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Fig 2: Code 32, Schematic, EGR System Error (Calif. Only)

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With ignition on and engine off, manifold pressure is equal to atmospheric pressure, and signal voltage will be high. This information is used by ECM as an indication of vehicle altitude, and is referred to as BARO. Comparison of this BARO reading with a known good vehicle with the same sensor is a good way to check accuracy of suspected sensor. Readings should be +/- .4 volt.

Code 33 is set if Gray wire is open, or if Gray/Red wire is shorted to voltage or shorted to Blue/Orange wire. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 33, Flow Chart, MAP Sensor Ckt./Signal Voltage High, Low Vacuum

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Fig 2: Code 33, Schematic, MAP Sensor Ckt./Signal Voltage High, Low Vacuum

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With ignition on and engine off, manifold pressure is equal to atmospheric pressure, and signal voltage will be high. This information is used by ECM as an indication of vehicle altitude, and is referred to as BARO. Comparison of this BARO reading with a known good vehicle with same sensor is a good way to check accuracy of suspected sensor. Readings should be +/- .4 volt.

Code 33 is set if Blue/Orange or Gray/Red wire is open or shorted to ground. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 33, Flow Chart, MAP Sensor Ckt./Signal Voltage Low, High Vacuum

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Fig 2: Code 33, Schematic, MAP Sensor Ckt./Signal Voltage Low, High Vacuum

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Using a Scan tester, observe block learn value at different RPM. Scan tester also displays the block cells, so the block learn values in each cell can be checked to determine when Code 44 may have been set. If the conditions for Code 44 exist, the block learn values will be about 150.

Ensure O2 sensor pigtail is not contacting exhaust manifold. Check for ground in wire between connector and sensor. Check for fuel contamination. Water causes a lean exhaust and can set Code 44. Check fuel pressure. System will be lean if fuel pressure is low. Check for exhaust leaks. If there is an exhaust leak, engine pulls outside air into the exhaust past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.

If the wrong fuel injectors are installed in vehicle, this will also cause a lean condition. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 44, Flow Chart, O2 Sensor Ckt./Lean Exhaust Indicated

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Fig 2: Code 44, Schematic, O2 Sensor Ckt./Lean Exhaust (VIN 5)

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Fig 3: Code 44, Schematic, O2 Sensor Ckt./Lean Exhaust (VIN 6)

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Code 45 or rich exhaust is most likely caused by improper fuel pressure, HEI shielding, canister purge, MAP sensor, TPS, O2 sensor contamination, or EGR system. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section. On VIN 5, if O2 sensor heater fails, Code 44 or Code 45 may set. To diagnose heater part of O2 sensor, refer to I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

Fig 1: Code , Flow Chart, O2 Sensor Ckt./Rich Exhaust Indicated

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Fig 2: Code 45, Schematic, O2 Sensor Ckt./Rich Exhaust (VIN 5)

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Fig 3: Code 45, Schematic, O2 Sensor Ckt./Rich Exhaust (VIN 6)

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