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

Chart A-2 - No 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. Battery ignition voltage is supplied to the light bulb. The Electronic 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 steady light suggests a short to ground in the light control CKT 419, or an open in 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 okay. 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". Each driver is part of a group of four, called "quad-drivers." Failure of one driver can damage any other driver in the set. Solenoid and relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "driver". See CHART C-1.