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Check engine light

my teenage son bought a 1990 gmc sierra 4.3 truck … accidentally purchased ethanol laced gasoline first fill-up and cel came on… what steps should i try first? autozone diagnostic code test?

Autozone.

By the way, 10% “ethanol laced” is now common and has no adverse effects on a modern engine beyong slightly lowered mpg. If he used 85% post back. He may have to purge the gas tank.

it was 10 should he put gas additive in with regular tank?

Yeah, that’s my thought. E85.

I’d either drain the tank or drive it non-stop until I could get another tank of fuel in there.

 That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here. The code should help pin it down. Most all gas today is 10% 

 It likely is not going to be a problem unless that 4.3 engine is a diesel.