With 4L80-E Automatic Transmission

Chart A-1 Diagnostic Chart:

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

CIRCUIT DESCRIPTION
There should always be a steady "Service Engine Soon" or "Trans" light with the ignition "ON" and engine "OFF". Switched battery voltage is supplied to the light bulb. The PCM/TCM will control the light and turn it "ON" by providing a ground path through circuit 419.

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

1. If the fusible link is open, refer to Engine Control Module / Diagrams for complete circuit.

2. Using a test light connected to 12 volts, probe each of the system ground circuits to be sure a good ground is present. refer to Engine Control Module / Diagrams for PCM/TCM pin locations of ground circuits.

DIAGNOSTIC AIDS
If the engine runs OK, check:
^ Faulty light bulb.
^ Circuit 419 open.
^ Gauges fuse blown. This will result in no brake warning light, oil or generator lights, seat belt reminder, etc.

If the engine cranks but will not run, check:
^ Continuous battery-fuse or fusible link open.
^ PCM/TCM "ECM-IGN" fuse open.
^ Battery circuit 440 to PCM/TCM.
^ Ignition circuit 439 to PCM/TCM open.
^ Poor connection to PCM/TCM.