Chart A-2 Won't Flash Code 12 Service Engine Light On

Chart A-2:

Wiring Diagram For Service Engine Soon Light:

CIRCUIT DESCRIPTION:

There should always be a steady "SERVICE ENGINE SOON" light when the ignition is "ON" and engine stopped. Battery ignition voltage is supplied to the light bulb. The ECM will turn the light on by grounding circuit 419. With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble code(s) stored in memory. A steady light indicates a short to ground in the light control circuit 419, or an open in diagnostic circuit 451.


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

1. If there is a problem with the ECM that causes a "Scan" tool to not read Serial data, then the ECM should not flash a Code 12. If a Code 12 does flash, be sure that the "Scan" tool is working properly on another vehicle. If the "Scan" tool is functioning properly and circuit 461 is OK, the PROM or ECM may be at fault for the "NO" ALDL symptom.
2. If the light goes "OFF" when the ECM connector is disconnected, then circuit 419 is not shorted to ground.
3. This step will check for an open diagnostic circuit 451.
4. At this point the "SERVICE ENGINE SOON" light wiring is OK. The problem is a faulty ECM or PROM. If a 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.

CAUTION: Not all SCAN testers are compatible with every system. A SCAN tool that produces faulty readings should NOT be used, and the problem should be reported to the tool manufacturer. Use of a faulty SCAN tool can result in improper diagnosis and unnecessary component replacement. Follow the proper instructions, located in the owners manual, for the specific SCAN tool being used.