How do you check if the engine thermostat is functioning?

If a thermostat is working, it should open at a specific temperature, which varies depending on the thermostat. Common temps are 195, 180 and 160. To check the thermostat, removed it from the engine, and place it in a pan of water with a thermometer. Heat the water to boiling and watch the thermostat. It should open before the water boils. Check the temperature at which it opens completely and see if that is satisfactory.

Anotehr WA is to start the eingine and let it idle. Obviously, if the engine overheats there is a problem. It could be a thermostat that is stuck closed or something else. If the engine does not warm up, or does not maintain a good operating temperature, then the thermostat is stuck open. If after ten minutes or so, the upper radiator hose is hot to the touch, then the thermostat is probably working properly.

Personally, I just change mine if there is any question if it is good or not. They are cheap. I often replace mine when I flush the cooling system, just like I change my cap and rotor every year.