Chart A-2 No Code 12 Service Engine Soon Light ON Steady

Chart A-2 - Won't Flash Code 12 "Service Engine Soon" Light On Steady:

Wiring Diagram for "Service Engine Soon" Light:

CHART A-2 - NO CODE 12 "SERVICE ENGINE SOON" LIGHT "ON" STEADY

Circuit Description:


There should always be a steady "Service Engine Soon" light when the ignition is "ON" and engine stopped. The Engine Control Module (ECM) will turn the light "ON" by grounding CKT 419 at the ECM.

With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble code(s) stored in memory. A dim but steady light when engine is running would indicate quad-driver failure due to a defective relay or solenoid. See test description No. 4. A steady light, with engine running, suggests a short to ground in the light control CKT 419 or diagnostic CKT 451.


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

1. If the light goes "OFF" when the ECM connector is disconnected, then CKT 419 is not shorted to ground. Also, check the connector terminals physically for proper contact at this time.

2. This step will check for an open diagnostic CKT 451.


3. At this point the "Service Engine Soon" light wiring is OK. The problem is a faulty ECM or PROM. The ECM is OK if a Code 51 is stored when the PROM is removed. Replace the PROM.

4. Solenoids and relays are turned "ON" or "OFF" by the ECM using internal electronic switches called "drivers." See CHART C-1.