Malfunction Indicator Lamp: Description and Operation

MIL Circuit:

This lamp is on the instrument panel and has the following functions:

^ It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.

^ It displays trouble codes stored by the ECM which help the technician diagnose system problems, when the diagnostic terminal is grounded with a spare fuse in the fuse box.

As a bulb and system check, the lamp will come "ON" with the key "ON" and the engine not running. When the engine is started, the lamp will turn "OFF." If the lamp remains "ON," the self-diagnostic system has detected a problem. If the problem goes away, the lamp will go out in most cases after 10 seconds, but a Trouble Code will remain stored in the ECM. When the lamp remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, follow the FLOW OF DIAGNOSIS.

The ECM and Malfunction Indicator Lamp (MIL) operates as follows:

The Malfunction Indicator Lamp (MIL) illuminates when the ignition switch is turned "ON" (but the engine is stopped) with the diagnosis switch terminal ungrounded regardless of the condition of EFI system. This is only to check the Malfunction Indicator Lamp (MIL) bulb and its circuit. If the EFI system is free from any faults after the engine starts (while engine is running), the Malfunction Indicator Lamp (MIL) turns "OFF." If any trouble exists, it may turn "OFF" but only until the ECM detects that fault. When ECM detects a trouble which has occurred in the above areas, it turns the Malfunction Indicator Lamp (MIL) "ON" while the engine is running to warn the driver of the occurrence of a fault and at the same time it stores the trouble code in ECM back-up memory. (The code is kept in memory even if the fault was only temporary and disappeared immediately. It is not erased unless the power to ECM is shut "OFF" for 20 seconds or longer.) The ECM also indicates trouble codes in memory by means of flashing of Malfunction Indicator Lamp (MIL) at the time of inspection (i.e. when the diagnosis switch terminal is grounded and ignition switch is turned "ON").