what is the cause flashing service engine light on 1997 GMC Suburban 1500

truck was doing just fine stopped to get some parts for another car came back out started it to leave and it was missing and the service engine light came on and now its on all the time and blinking.

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2 answers
Engine Misfire
Spark Plug Wire
Loose Gas Cap
Three cases can cause the flashing service engine light.
Flashing Check engine light is caused by a misfire. Its to alert you, that you need to get it checked out ASAP. there are a bunch of things that can cause a misfire on these vehicles: Distributor, distributor cap, wires, plugs, injector, cylinder head issues, etc...

You would simply need to have it diagnosed by a competent repair shop.