The thermostat is a valve that keeps coolant circulating in the engine until it reaches normal running temperature. When this happens, the thermostat opens and allows coolant to flow to the radiator. If the thermostat fails to open, the engine will overheat. If the thermostat sticks open, the engine will take longer than normal to reach operating temperature, causing poor heater performance, higher exhaust emissions, and on some vehicles, the check engine light to illuminate.
To replace the thermostat, the cooling system is drained, the radiator hose is removed from the thermostat housing, and the housing is removed from the engine. The thermostat and its housing gasket are replaced, the housing and hose are reassembled, and the cooling tank is filled with fresh coolant.