it might not be your thermostat, it could be the temperature sending unit. but your thermostat could be stuck open which wont hurt anything, you just wont get good heat at all. and it would be almost impossible to over heat

check the sending unit, but turning on your heat, if its hot as hell then its probably the sending unit. or you could check the upper radiator hose to see if it gets hot. mine did the same thing all the time and it was the sending unit. now its all fine