Service engine light on 2004 GMC Envoy

Twice, I drove more then 200 miles at one time, my service engine, and the reduce engine power light comes on. I have taken it to the GMC repair shop several times and they can't find a "code". I let her sit for 12 hours or more the light is off and she runs ok. WHAT CAN BE THE PROBLEM BE?

Asked by for the 2004 GMC Envoy
If the Service Engine Soon/Check Engine light illuminates the on board computer in your Envoy stores a code that tells the technician what diagnostic trouble shooting path to follow. At our shop we have three scanners to check for trouble codes. Some scan tools are better than others and some are better with one make of car than another. I would have to say bring your GMC to another shop that specializes in GMC. Perhaps there scan tool can interpret the data in your Envoy. There are several modes of diagnostic capabilities, Mode 6 should show your problem to someone vry familiar with GMC..
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