Circuit Description

There should always be a steady "SERVICE ENGINE SOON" ("SES") light when ignition is turned on with engine off. Battery voltage supplied directly to illuminate the bulb. The ECM will control the light and turn it on by providing a ground path through circuit No. 419 to ECM. See Figure

With diagnostic terminal grounded, light should flash a Code 12, followed by any trouble code(s) stored in memory.

A steady light indicates a short to ground in light control Circuit No. 419, or an open in diagnostic circuit No. 451. Refer to Fig. Figure