Intermittent Check Engine Light

I have a 1988 Chevy Silverado with a 350 cu inch. I have replaced the engine control module a couple of times. My check engine light comes on after the truck has been running for 20 minutes and stays on. The truck runs fine and after it is shut off and restarted the light is off. I suspect it is a sensor but do not know what to do to fix the issue. I have taken it to a mechanic but there is limited diagnostics on this model. Any ideas?

You can pull the codes yourself using the CEL as your indicator.

Look below the dash area to the left of the steering wheel for a small data connector, it should have a dozen or so contacts. With the engine OFF, take a paper clip and short the two upper-rightmost pins together (someone double-check this). Now turn the ignition to ON, but do not start the engine.

The CEL will start blinking; it should go blink-pause-blink-blink, indicating Code 12 (1, then 2) meaning it’s in diagnostic mode. It’ll do this three times, and then if there are any codes stored it will flash the CEL to indicate what they are (example, code 34 would be blink-blink-blink-pause-blink-blink-blink-blink).

Post back what you get. You’ll have the most success if this is done immediately after you’ve driven the truck and its CEL came on.

“1988 Chevy” " I have taken it to a mechanic but there is limited diagnostics on this model."

Did the 1988’s have that code capability?

Here’s a guide on how to retrieve the codes and the code definitions.