The 2008 GMC Envoy indoor regulator is in the indoor regulator lodging on the lower coolant line. The indoor regulator estimates the temperature of the coolant going through the line,and from that point it can figure out what temp the motor is running at. In the event that this part isn't working then the motor risks overheating,which is awful for the vehicle. In the event that a motor overheats it will risk distorting or breaking the head gasket,which will cost thousands to fix over the long haul. Assuming You get a check motor light letting you know that the indoor regulator isn't working then sort it out immediately. It is around 200 for a full substitution so You would rather not pause