Chart A-2 Code 12 Will Not Flash

Fig. 063 - Chart A-2 - Code 12 Will Not Flash:

Fig .064 - Wiring Diagram for Chart A-2 Code 12 Will Not Flash. VIN 1:

CIRCUIT DESCRIPTION:
There should always be a steady "Service Engine Soon" light when the ignition is "ON" and the engine is "OFF." Battery voltage is supplied directly to the There should always be a steady "Service Engine Soon" light when the ignition is "ON" and the engine is "OFF." Battery voltage is supplied directly to the light bulb. The Electronic Control Module (ECM) will control the light and turn it "ON" by providing a ground path through CKT 419 to the ECM.
With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble code(s) stored in memory. A steady light suggests a short to ground in the light control CKT 419, or an open in diagnostic CKT 451.

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. If the light turns "OFF" when the ECM connector is disconnected, then CKT 419 is not shorted to ground.

2. This step will check for an open diagnostic CKT 451.

3. At this point, the "Service Engine Soon" light wiring is OK. The problem is a faulty ECM or PROM. If Code 12 does not flash, the ECM should be replaced using the original PROM. Replace the PROM only after trying an ECM, as a defective PROM is an unlikely cause of the problem.

Never apply full battery voltage across the injector terminals, as the coil will be instantly destroyed.