Mode Description
Diagnostic ModeWhen the diagnostic terminal is grounded with the ignition "ON" and the engine "OFF," the system will enter what is called the Diagnostic Mode. In this mode the ECM/PCM/VCM will:
1. On certain vehicles with a 12 pin DLC connector (pin "B" included), display a DTC 12 by flashing the MIL (Service Engine Soon) (indicating the system is operating correctly). If any additional DTC's are set, it will flash those DTC's. Each DTC will be flashed three times, then DTC 12 will be flashed again.
2. Energize most system controlled relays and solenoids except fuel pump relay. This allows checking circuits which may be difficult to energize without driving the vehicle and being under particular operating conditions.
3. The IAC valve moves to its fully extended position on most models, blocking the idle air passage. This is useful in checking the minimum idle speed.
Field Service Mode
If the diagnostic terminal is grounded with the engine running, the system will enter the Field Service mode.
In "Open Loop" the MIL (Service Engine Soon) flashes two and one-half times per second.
In "Closed Loop," the light flashes once per second. Also, in "Closed Loop," the light will stay "OFF" most of the time if the system is running lean. It will stay "ON" most of the time if the system is running rich.
While the system is in Field Service Mode, new codes cannot be stored and the "Closed Loop" timer is bypassed.