Malfunction Indicator Lamp: Description and Operation

PURPOSE
Illuminates when a influencing occurs that will affect emission control performance. Flashes trouble codes (DTC) during testing operations.

LOCATION
In the instrument cluster.

OPERATION
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 diagnosis and repair.

Another function of the "Check Engine" light is 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.

In "Closed Loop", the light will stay "OFF" most of the time if the system is running too lean. It will stay "ON" most of the time if the system is too rich.

While in Field Service Mode, new trouble codes cannot be stored in the ECM memory and the "Closed Loop" timer is bypassed.