Car Doctor Q&A: 2002 GMC Yukon Check Engine Light Woes

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After 232,000 miles, a lot of 2002 GMC Yukon owners would opt for a strip of black electrical tape to fix a meddlesome Service Engine Soon light. But learning what the issue is and fixing it the right way is the way to go, says John Paul.

Q. I am the original owner 2002 GMC Yukon with 232,000 miles on it. For several years now the “service engine soon” light has been on. It is not on all the time, and sometimes it flashes and sometimes it is on steadily.  Both the dealer and private mechanics have read a code that indicates “moderate misfire”.


Unfortunately no one has been able to accurately diagnose the problem and correctly fix it. It still starts and runs great and the gas mileage does not seem to be suffering.  I do notice some slight roughness at idle and of course the annoying trouble light. Not sure if this is related but I also notice a slight “pulsing” in both the vehicle’s interior lights and headlights.  While in for service recently the mechanic suggested that the O2 sensors were “lazy” and ought to be replaced.  Any thoughts/suggestions you have would be greatly appreciated!

A. I think you may be referring to a code for a random misfire, although there are many possibilities. The check engine light is also telling part of the story. When the check engine light is flashing this indicates an engine misfire. This misfire can cause permanent damage to the catalytic converter.

At this point my priority would be to carefully look for issues that could cause a random misfire to prevent having to replace the catalytic converter. Just looking at codes is only part of the story; a good technician will look at fuel trim to determine if there is a vacuum leak or other fuel related issues .

The first place I would look would be a intake manifold leak. This leak is best checked with a smoke machine. This tool allows the technician to pump synthetic smoke into the air intake and look for leaks. Of course it is also possible that at 232,000 miles the engine could be just wearing out and would warrant a compression test to verify overall engine condition.


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