Malfunction Indicator Lamp: Description and Operation

PURPOSE
- Informs the driver that a malfunction is occuring in the engine control system.
- Displays trouble codes.
- Indicates "Open Loop" or "Closed Loop" operation.

LOCATION
In the instrument cluster.

OPERATION
- System Monitor Mode
When the system detects a fault in a monitored component and/or circuit, the light will come on and stay on (while the engine is running), alerting the driver to seek service. With intermittent problems, the light may come on and then go out, but a trouble code will be stored in ECM memory if correct parameters are met. The technician can then extract trouble codes from the ECM that will aid in the performance of efficient troubleshooting and repair.
As a self-check feature, the lamp should illuminate with the key ON and engine OFF.

- Field Service Mode
When the diagnostic terminal is grounded with the engine running, the system will enter this mode and indicate whether it is in "Open Loop" or "Closed Loop." If in "Open Loop," the "Check Engine" light flashes two and one-half times per second. In "Closed Loop," the light flashes once per second.

- Trouble Code Extraction
When extracting diagnostic trouble codes (DTC), the light will flash a numeric code. The code is determined by the number of flashes and at what frequency.