Diagnostic Mode Description

When the diagnostic terminal is grounded with the ignition "ON" and the engine "OFF," the system will enter the diagnostic mode. In this mode, the control module will:

1. Display a Diagnostic Trouble Code (DTC) 12 by flashing the Malfunction Indicator Lamp (MIL) "Service Engine Soon" (indicating the system is operating properly).
2. Display any stored DTC(s) by flashing the Malfunction Indicator Lamp (MIL) "Service Engine Soon." Each DTC will be flashed three times, then DTC 12 will be flashed again.
3. Energize all control module controlled relays and solenoids except the fuel pump relay. This allows checking circuits which may be difficult to energize without driving the vehicle and being under particular operating conditions.
4. The Idle Air Control (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.