Diagnostic System

The ECM of the Closed Loop Emission Control system is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, the ECM will light the amber "CHECK ENGINE" lamp located on the instrument panel. At the same time, a corresponding trouble code is stored in ECM memory. Malfunctions are recorded as "hard failures" or "intermittent failures".

"Hard failures" cause "CHECK ENGINE" lamp to glow and remain on until malfunction is repaired. If the "CHECK ENGINE" lamp comes on and remains on during vehicle operation, cause of malfunction MUST be determined.

Fig 1: Location of Diagnostic Terminal on Electronic Control Module
Courtesy of GENERAL MOTORS COMPANY

View of ECM mounted position behind glove compartment.
"Intermittent failures" allow lamp to flicker or go out, 10 seconds after fault goes away. However, the associated trouble code will be retained in ECM memory. "Intermittent failures" may be sensor related. If a sensor fails, ECM will use a substitute value in its calculations to continue engine operation. In this condition, service is not mandatory, but loss of driveability may result.

As a bulb and system check, the "CHECK ENGINE" lamp will glow when ignition switch is turned on and engine is not running. When engine is started, the lamp should go out after 1-4 seconds. If not, a malfunction has been detected in the Closed Loop Emission Control system.

NOTE: Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds; others require operation for 5 minutes or longer.

Diagnose the Closed Loop Emission Control system in the following order:

Ensure all engine systems NOT related to the system are fully operational. Do not proceed with testing unless all other problems have been corrected. Ensure that all electrical and vacuum connections are correct and in good condition.
Enter diagnostic mode and record trouble codes flashed by "CHECK ENGINE" lamp. Exit diagnostic mode. Distinguish between "hard" and "intermittent" codes.
If trouble codes were displayed, go to DIAGNOSTIC CIRCUIT CHECK CHART . Follow instructions given there.
If no trouble codes were recorded, go to DRIVER COMPLAINT CHART and follow instructions given there.
After any repairs are made, perform SYSTEM PERFORMANCE CHECK . Clear any trouble codes.