Q: Service Engine Light comes On on 2002 GMC Envoy

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Service Engine Soon light came on, took to Autozone and they ran a diagonistics which said number 3 cylinder misfiring. They told me to put some lucas fuel injection cleaner in it run it and if it didn't work change the spark plugs. The car has over 100,000 miles on it. Added the fuel injection cleaner ran for 2.5 hours and 200 miles. Light was gone, however when I was parking the vehicle it came back on. Next day replaced spark plugs, light out. Ran for 20 minutes light on again. Now what do I check?
(2) Answers
you have coils that fire the plugs that may need to be replaced. take it to shop for a real diag by a pro, not a salesmann that is not a tech.

I had this problem (Mine said 2) I swapped it with 1 and 1 errored, I replaced the coil pack and it worked fine.