With 4L80-E Automatic Transmission

No ALDL Data Chart A-2:

Service Engine Soon Lamp Circuit:

Circuit Description:
There should always be a steady "Service Engine Soon" light when the ignition is "ON" and engine stopped. Battery voltage is supplied to the light bulb. The PCM will turn the light "ON" by grounding CKT 419 at the PCM.

With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble codes 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 there is a problem with the PCM that causes a Tech 1 "Scan" tool to not read serial data then the PCM should not flash a code 12. If Code 12 does flash, be sure that the "Scan" tool is working properly on another vehicle. If the "Scan" is functioning properly and CKT 1061 is OK, the PROM/MEM-CAL or PCM may be at fault for the no ALDL symptom.

2. If the light goes "OFF" when the PCM connector is disconnected, then CKT 419 is not shorted to ground.

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

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