Malfunction Indicator Lamp: Description and Operation

This system incorporates self-diagnostic capability that is monitored via a dash mounted Malfunction Indicator Light. 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.

Another function of the Malfunction Indicator 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 Malfunction Indicator Light flashes two and one-half times per second. In "Closed Loop," the light flashes once per second.

As a diagnostic tool, the light will stay "OFF" most of the time in "Closed Loop" if the system is running lean. It will stay "ON" most of the time if the system is rich.

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