Malfunction Indicator Lamp: Description and Operation

Fig 1 Data Link (Diagnostic) Connector:

MALFUNCTION INDICATOR LAMP (MIL)
As a functional test, the Malfunction Indicator Lamp (MIL) illuminates at key-on before engine cranking. Whenever the Powertrain Control Module (PCM) sets a Diagnostic Trouble Code (DTC) that affects vehicle emissions, it illuminates the MIL. If a problem is detected, the PCM sends a message over the PCI Bus to the instrument cluster to illuminate the lamp. The PCM illuminates the MIL only for DTC's that affect vehicle emissions. The MIL stays on continuously when the PCM has entered a Limp-In mode or identified a failed emission component or system. The MIL remains on until the DTC is erased.

Also, the MIL either flashes or illuminates continuously when the PCM detects active engine misfire. Refer to Misfire Monitoring.

Additionally, the PCM may reset (turn off) the MIL when one of the following occur:
- PCM does not detect the malfunction for 3 consecutive trips (except misfire and fuel system monitors).
- PCM does not detect a malfunction while performing three successive engine misfire or fuel system tests. The PCM performs these tests while the engine is operating within 375 RPM of and within 10 % of the load of the operating condition at which the malfunction was first detected.

DIAGNOSTIC TROUBLE CODES
A Diagnostic Trouble Code (DTC) indicates the PCM has recognized an abnormal condition in the system.

The preferred and most accurate method of retrieving a DTC is by using the DRB scan tool. The scan tool supplies detailed diagnostic information which can be used to more accurately diagnose causes for a DTC.

Remember that DTC's are the results of a system or circuit failure, but do not directly identify the failed component or components.

BULB CHECK
Each time the ignition key is turned to the ON position, the malfunction indicator (Check Engine) lamp on the instrument panel should illuminate for approximately 2 seconds then go out. This is done as a bulb check.