Check engine light does not come on..

[bginnc]

1996 Silverado 350 engine. When I turn ignition key on, all system check lights come on as they should except "check engine soon". If I connect code reader the light comes on and will come on the next time ignition is turned on but only one time. The second time you try it, it does not. What would cause this? Thanks

[777stickman]

The SES lamp is powered thru the "gauges" fuse in the run & start position. Since all other lights and gauges are responding correctly this fuse is good. The VCM controls the ground circuit at the C4 VCM connector (labeled "black" on the VCM). Since you can turn the SES on or off with the scan tool makes it very hard to diagnose.

I would try this: Next time the lite does not come on in the "run" position, get out and wiggle the wires at the VCM connector C4 (black) while someone watches the SES lite.

Let us know.

[bginnc]

Thanks, I will check this out.

[Schurkey]

Computer is capable of turning on the light. Seems predictable, not random.

First Guess: Defective ECM.