Check engine light on 2003 gmc

The amber check engine light came on about 6 months ago and I noticed that the heat gauge never got to the full operating temp… Anyway, a mechanic checked the code on a scanner,and sure enough it showed a thermostat problem- - -so, I had the thermostat replaced, and now this past month, the check engine light came on again and won’t go out- - -is it possible that it was a bad thermostat that replaced the old one. Also, at times the heat gauge will reach normal (not often) and the light will go out. It’s still on as of today, though.

It is always nice if you provide the actual code (Like P0123). You car has a device called a thermostat that opens and closes to keep the car not too cold and not too hot. There will be at least one and maybe more sensors that measure the temperature, one of which is responsible for turning on that CEL. The full error code will help ID what might be the problem. In addition you could have blocked a hose or other problems.

Now of course if I read your message properly, the current light could have nothing to do with the engine temperature, it could be a misfire, bad gas cap or any of a hundred items or sensors.  

Many auto part stores will check the code for free.  Find one and then bring the code {Remember codes are like [P1234]}  and post it as a reply to your original message.

Thanks for your prompt reply; the mechanic I went to advised me to go to the GM dealer (said it was most probably an “electric” issue, etc), though I’m a;ways leery of going to a dealer ($$$$$$$). I do have a letter from GM stating that for that particular year (2003 Sierras) there was an dashboard/instrument/gauge problem and that the instrument “cluster” may have to be replaced (at no charge) if I experience this problem with erratic gauge readings, et…don’t know whether this issue fits the situation. Might be worth a try. Thanks again!