2004 GMC Envoy check engine light on

It should be pretty easy to check if the thermostat is bad, you say it doesn't overheat, is your vehicle equipped with a guage? If so does the the temperature rise into the normal range when driving, if not the thermostat may be stuck open, if it does rise to normal you may have a problem with your coolant temperature sensor. Both of these repairs are relatively inexpensive to have done, if you are trying to save the diagnostic fee you could probably replace both with parts from a do it yourself parts house. Remember coolant is hot and may be under pressure so please let your vehicle cool completely prior to opening the cooling system or removing the radiator cap. You may have to have the trouble codes cleared from memory after completing repairs.