Chart A-6 Service Engine Soon Always ON or No Code 12

Chart A-6 "Service Engine Soon" Light "On" At All Times Or Won't Flash Code 12:

Wiring Diagram - "Service Engine Soon" Light:

CHART A-6

"SERVICE ENGINE SOON" "ON" AT ALL TIMES OR WON'T FLASH CODE 12


Circuit Description:

When the engine is started, the ECM grounds terminal "G" to turn off the "Service Engine Soon" light. It alternately grounds and opens it to flash a code.


Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. This step checks for short to battery voltage in wire to terminal "C" or faulty lamp driver. Normal voltage reading is 9-11 volts.

2. This step checks to see if problem is related to the ECM or the lamp driver. Normally, grounding terminal "C" should turn lamp "OFF." If it does, the problem is related to the ECM and its wiring. If not, it is related to the lamp driver and its wiring.

3. Grounding terminal "G" at ECM and light "ON" indicates an open in CKT 487 to terminal "C" of lamp driver. Normally, grounding Terminal "G" should turn lamp "OFF."


4. This step checks for open CKT 451 from ECM to test terminal in ALDL connector. The lamp should flash Code 12, when terminal "5" is grounded.

5. Checks for proper voltage supply to ECM. Both should read over 9 volts. Terminal "C" is ignition, and terminal "R" is constant battery for long term memory.

6. Checks for a bad ground to ECM - terminal "A" and "U" are connected together in the ECM.

7. This step distinguishes between a faulty ECM and PROM. Normal response is for Code 51 to flash, even though the PROM is not installed in the ECM. If it doesn't, it means that the ECM is faulty.