Entering Diagnostic Modes

Fig. 2 Trouble Code Test Lead Identification:

If the Diagnostic "test" terminal is grounded with the ignition "ON" and the engine stopped, the system will enter Diagnostic Mode. In this mode the ECM will:

1. Display a code "12" by flashing the "CHECK ENGINE" light (indicating the system is operating). A code "121" consists of one flash, followed by a short pause, then two flashes in quick succession. This code will be flashed three times. If no other codes are stored, code 12 will continue to flash until the Diagnostic "Test" terminal is ungrounded.

NOTE: Codes can only be obtained with the engine stopped. Grounding the "Test" terminal with the engine running gives the "Field Service Mode".

2. Display any stored trouble codes by flashing the "CHECK ENGINE" light. Each code will be flashed three times, then code "12" will be flashed again.

If a trouble code is displayed, a Diagnostic Chart is to be used to find a problem.

3. Energize all ECM controlled relays and solenoids (with some exceptions).

On a fuel injected engine, the IAC valve also moves back and forth or is fully extended depending on the calibration.

4. Additional trouble codes.

5. Take out the open loop timer.