Chart 1 On-Board Diagnostic System Check

Diagnostic Chart 1:

Wiring Diagram:

After the visual/physical underhood inspection, the on-board diagnostic system check (OBD system check) is the starting point for all diagnostic procedures or finding the cause of an emissions test failure.


Since a Scan tool is required to perform any Diagnostics this is the starting point. The PCM and the ABS (if equipped) share the same data link referred to as the class II data bus. The air bag system uses its own data link which is not connected to the class II data bus.

- When the ignition is turned On the PCM will perform a bulb test on the SERVICE Lamp, the Low Coolant/Hot Lamp and MIL (SERVICE ENGINE SOON) lamp. The lights should stay On 2-3 seconds after the engine is started and turned Off. If any of the lamps remain On, a problem and possible DTC will exist.

- If data can be read, the engine starts and runs OK, and there are no DTCs stored, then diagnosis will depend on the symptoms or customer complaint.

- Circuit 1140 (battery keep alive) has 12 volts applied to it at all times. Information such as diagnostic trouble codes, long term fuel correction, spark adaptives, etc., requires power to retain their learned values.

- Circuit 551 (ground) is grounded to the engine at the top front bell housing stud.

Flash power and flash return are used when reprogramming the PCM.