Initial Inspection and Diagnostic Overview

Trouble Code Test Leads:

CAUTION: To prevent ECM damage, the key must be "OFF" when disconnecting or reconnecting power to the ECM.

Reading Trouble Codes:

READING TROUBLE CODES

A diagnostic circuit check confirms that the on board self diagnostics are functioning and displays trouble codes that may be stored. When the ignition is turned "ON," the "Check Engine" lamp should come on and then go out when the engine is started and no trouble codes are stored. If the light remains on, the system has detected a fault.
Two test leads located near the ECM connector are used to activate the diagnostic mode. To enter diagnostic mode, connect the two leads together with the ignition switch "ON" and the engine "OFF." The ECM will display codes by flashing the "Check Engine" lamp. The first code to display should be a code 12 to indicate that the self diagnostics are functioning. Code 12 is displayed by a flash, followed by a short pause, then two flashes, followed by a longer pause. If other codes are present, they will display in the same manner after code 12 is displayed three times. For example, code 23 is displayed by two flashes at regular intervals, followed by a short pause, then three flashes at regular intervals, followed by a longer pause. Each code is displayed three times followed in numerical order by any others in the ECM memory. When all codes have been displayed, code 12 will be repeated. If no codes are stored, code 12 will display repeatedly until the diagnostic test leads are disconnected. Disconnect test leads before starting engine.

CLEARING TROUBLE CODES

After faults have been corrected, it is necessary to clear trouble codes from the ECM memory by interrupting its power supply for at least 10 seconds. To accomplish this, either remove the main 60A fuse or disconnect the negative battery cable.

TROUBLE CODE DESIGNATIONS

Code 12: No ignition reference pulses to ECM. Only flashes while fault is present and before engine is started.
Code 13: Oxygen sensor circuit.
Code 14: Shorted coolant sensor circuit.
Code 15: Open coolant sensor circuit.
Code 21: Idle switch and MAP sensor failure.
Code 22: Fuel cut solenoid circuit open or grounded.
Code 23: Duty solenoid circuit open or grounded.
Code 25: Air management valve circuit open or grounded.
Code 26: Canister VSV circuit open or grounded.
Code 27: Constant high voltage from VSV to ECM.
Code 31: No ignition reference pulses to ECM after 10 seconds at part throttle, under load.
Code 32: EGR system failure and sensor circuit failure.
Code 34: EGR gas sensor circuit failure.