My 2005 Envoy through a P0128 code a couple weeks ago which came back as a thermostat code. I reset the code with my Blu Drive reader and it hasnt come back since. I also have been watching the real time temp readings, the car warms up quick, heat is nice and hot and the temp gets to around 185, drops to 176 and stays consistent at 185, the dash gauge reads at the 210 level and since its really cold out, will drop if I am stopped idling and warm back up when driving. I also pulled the radiator cap when cold and there is no movement of the coolant which to me would presume the thermostat is working. Normally I would just replace the thermostat and be done but this car is a pain and requires removal of the alternator and other parts to get at the thermostat so I dont want to mess with it unless it really is a problem. Should I consider the code to be a 1 time thing and go with it or just take on the job and get it done? Thanks!