Malfunction Indicator Lamp Operation

The Malfunction Indicator Lamp (MIL) is intended to alert the driver of certain malfunctions in the EEC-IV system. The MIL is a light in the dash that reads "CHECK ENGINE". If the light is ON, the driver of the vehicle should take the car in for service as soon as possible.

NOTE: It is not necessary to immediately turn the engine OFF and have the vehicle towed when the MIL comes ON.

The MIL will come ON while the engine is operating in Failure Mode Effects Management (FMEM) or Hardware Limited Operation Strategy (HLOS) modes. The light will stay ON for at least 10 seconds, then stay ON as long as the fault causing it is present. If the MIL flashes quickly (less than 10 seconds), the MIL circuit should be checked for concerns.

In FMEM mode, the Powertrain Control Module (PCM) is receiving a sensor signal that is outside the limits set by the calibration strategy. In this mode, the PCM uses an alternate engine control strategy to maintain reasonable vehicle operation in spite of the fault. The Diagnostic Trouble Code (DTC) associated with this fault is stored in Keep Alive Memory (KAM). If the fault is no longer present, the light will turn OFF and the vehicle will return to the normal vehicle strategy. The DTC stored when the light was ON is kept in Continuous Memory for the next 80 warm-up cycles and then erased. This Continuous Memory DTC can be accessed by running the Key ON Engine OFF Self-Test.

HLOS mode is used when the system fault(s) is (are) too extreme for the FMEM mode to handle. In HLOS mode, all software operations have stopped and the PCM is running on hardware control only. The default strategy for this mode has a minimal calibration strictly to allow the vehicle to operate until it can be serviced.

NOTE: In HLOS mode self-test DTCs will not be output.

HOW THE MALFUNCTION INDICATOR LAMP OPERATES

System OK
The MIL is turned ON as a bulb check when the ignition key is first turned ON. The EEC-IV processor turns the bulb OFF as soon as it receives the PIP (cranking) signal.

System Not OK
If the MIL should remain ON after the vehicle has started, run Quick Test and service any DTCs. If Self-Test has pass DTCs, the MIL is always ON, and the vehicle has no drive symptoms, go to the diagnostic routines index.

If the MIL never comes on, go to the diagnostic routines index. With EEC (OBD I System)

NOTE: When in self-test, the MIL is not limited to continuous memory codes and will also flash other DTCs. Also, continuous memory codes are erased from continuous memory if the original fault has not occurred in the last 80 warm-up cycles (40 cycles on some applications).