Chart A-2, Won't Flash Code 12 (SES Light On Steady)

The SERVICE ENGINE SOON (SES) light should illuminate with ignition on and engine off. The ECM controls the SES light and turns it on by providing a ground path through light control circuit. With the diagnostic terminal grounded, the SES light should flash a Code 12, followed by any trouble codes stored in memory.

A steady light suggests a short to ground in the SES light control circuit, ECM terminal A5, or an open in diagnostic circuit, ECM terminal A9. A steady but dim SES light indicates failure of quad-driver.

1. If there is a problem with ECM, causing scan tester to not read serial data, the ECM should not flash Code 12. If Code 12 is flashing, check ECM terminal A5 for short to ground. If Code 12 does flash, ensure scan tester is working properly by testing it on a known good vehicle. If scan tester is working properly and Serial Data circuit is okay, terminal A5, the MEM-CAL or ECM may be the cause of NO ALDL fault.
2. If SES light goes off when ECM connector is disconnected, light control circuit is not shorted to ground.
3. This step checks for an open in diagnostic test circuit.
4. At this point, the SES light wiring is okay. The problem is a faulty MEM-CAL or ECM. If Code 12 does not flash, the ECM should be replaced using the original MEM-CAL. Replace the MEM-CAL only after trying another ECM, as faulty MEM-CAL is usually not the cause of this fault.

Fig 1: Chart A-2 Schematic - SES Light On Steady

Courtesy of GENERAL MOTORS CORP.

Fig 2: Chart A-2 Flow Chart - SES Light On Steady

Courtesy of GENERAL MOTORS CORP.