Malfunction Indicator Lamp: Description and Operation

Data Link Connector:

As a functional test, the MIL (check engine) 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 to the instrument cluster to illuminate the lamp. The PCM illuminates the MIL only for DTC's that affect vehicle emissions. There are some monitors that may take two consecutive trips, with a detected fault, before the MIL is illuminated. The MIL stays on continuously when the PCM has entered a Limp-In mode or identified a failed emission component. Also, the MIL either flashes or illuminates continuously when the PCM detects active engine misfire.

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.