With 4L80-E Automatic Transmission

Chart A-2 - PCM/TCM:

No Service Engine Soon/Trans Light Chart A-1:

CIRCUIT DESCRIPTION
There should always be a steady "Service Engine soon"/"Trans" light when the ignition is "ON" and engine stopped. Battery ignition voltage is supplied to the light bulb. The PCM/TCM will turn the light "ON" by grounding circuit 419.

With the "diagnostic" terminal grounded, the light should flash a DTC 12, followed by any trouble code(s) stored in memory.

A steady light suggests a short to ground in the light control circuit 419 or an open in diagnostic circuit 451, or no engine speed sensor signal.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. If there is a problem with the PCM/TCM that causes a "Scan" tool to not read serial data, then the PCM/TCM 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 circuit 1061 is OK, the PROM or PCM/TCM may be at fault for the NO DLC symptom.

2. If the light goes "OFF" when the PCM/TCM 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"/ "Trans" light wiring is OK. The problem is a faulty PCM/TCM PROM. If Code 12 does not flash, the PCM/TCM should be replaced using the original PROM. Replace the PROM only after trying a PCM/TCM, as a defective PROM is an unlikely cause of the problem.