Malfunction Indicator Lamp: Description and Operation

PURPOSE
The Malfunction Indicator Lamp (MIL) indicator is in the instrument panel cluster assembly and has the following functions:
^ It informs the driver that a malfunction has occurred and that the vehicle should be taken for service as soon as reasonably possible.
^ It displays Diagnostic Trouble Codes (DTCs) stored by the Engine Control Module (ECM) which helps the technician diagnose system malfunctions.

OPERATION
As a bulb and system check, the Malfunction Indicator Lamp should be lit with the ignition switch in the "ON" position (engine not running). When the engine is started, the MIL will go out. If the MIL remains lit, the self-diagnostic system has detected a malfunction. If the fault goes away, the MIL will go out immediately but a corresponding DTC will be stored in the memory of the ECM.

ECM UNABLE TO STORE DTC 41 IN MEMORY
DTC 41 (Ignition Signal Circuit), however, is an exception to this statement and is not stored in ECM memory. Any memory of DTC 41 is erased as soon as the ignition switch is turned to the "LOCK" position. In order to detect an active DTC 41, the engine must be cranked for 3 seconds and the Tech 1 scan tool diagnostic computer connected with the ignition switch left in "ON." If the ignition switch is turned to "LOCK" at any time during this procedure, a present DTC 41 will be lost.

READING TROUBLE CODES
For DTC retrieval information, refer to Computer and Control Systems / System Diagnosis / Procedures / Vehicle On-Board Diagnostics / Accessing and Reading Diagnostic Trouble Codes.

MIL "ON" WITH ENGINE RUNNING
When the MIL remains lit while the engine is running, or when a malfunction is suspected due to a driveability or emissions malfunction, the On-Board Diagnostic System Check must be performed. Refer to Computer and Control Systems / System Diagnosis / Procedures for this procedure. This check will expose malfunctions which may not be detected if other diagnostics are performed prematurely.