With 4L60-E Automatic Transmission

Chart A-2 - PCM:

DLC Circuit PCM:

CIRCUIT DESCRIPTION
There should always be a steady "Service Engine Soon" light when the ignition is "ON" and engine "OFF". Switched battery voltage is supplied to the light bulb. The Powertrain Control Module (PCM) 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 codes 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
Numbers below refer to circled numbers on the diagnostic chart.

1. If there is a fault with the PCM that causes a scan tool to not display serial data then the PCM should not display a DTC 12. If DTC 12 does display, be sure that the scan tool is working properly on another vehicle. If the scan tool is functioning properly and circuit 1061 is OK, the PROM or PCM may be at fault for the NO DLC symptom.

2. If the light goes "OFF" when the PCM 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 PROM. If DTC 12 does not display, the PCM should be replaced using the original PROM. Replace the PROM only after trying an PCM. A defective PROM is an unlikely cause of the problem.