Malfunction Indicator Lamp: Description and Operation

PURPOSE
The Malfunction Indicator Lamp (MIL) indicator is in the instrument cluster 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 help the technician diagnose system malfunctions.

OPERATION
Check Function
As a bulb and system check, the MIL should be lit with the ignition switch in the "ON" position (engine not running). When the engine is started, the MIL will go out.

Malfunction Present
If the MIL remains lit with the engine running, the self-diagnostic system has detected a malfunction. If the malfunction is no longer detected, the MIL will go out immediately but a corresponding DTC will be stored in the memory of the ECM.

Non Storeable DTCs
The following DTCs are not stored in the memory of the ECM once the malfunction is no longer detected: DTC 16, DTC 43 and DTC 51.

MIL Remains Lit
If the MIL remains lit when the engine is running, or when a malfunction is suspected due to a driveability or emission malfunction, the On-Board Diagnostic System Check must be performed. Refer to Powertrain Management / Computers and Control Systems / System Diagnosis for this procedure. This check will expose malfunctions which may not be detected if other diagnostics are performed prematurely.