Malfunction Indicator Lamp: Description and Operation

PURPOSE
The Malfunction Indicator Lamp is used to indicate immediate service of the engine control system is required.

The lamp will illuminate when the Powertrain Control Module (PCM) detects an open or shorted circuit, absence of current flow in an output device, or if the oxygen sensors are not switching.

OPERATION
The Malfunction Indicator Lamp illuminates when the ignition key is turned to the ON position and stays lit for 3 seconds as a bulb test. The bulb check operation is controlled by the PCM.

If a problem is detected, the Powertrain Control Module (PCM) sends a message, over the CCD Bus, to the Body Control Module (BCM).

The BCM interprets this message and sends a signal to the instrument cluster to illuminate the lamp.

When the lamp is illuminates it indicates the PCM has entered limp in mode. When in limp in mode, the PCM substitutes the reading from a functioning input for the failed input value.

Signals that can set Malfunction Indicator Lamp are battery voltage input, charging system, manifold Absolute Pressure sensor (MAP), Throttle Position Sensor (TPS), coolant temperature sensor.

The Malfunction Indicator Lamp can also be used to display fault codes that have been stored in the PCM memory due to a system failure.