The Thermostat

The thermostat is a simple valve that controls coolant flow based on temperature. When the engine is cold, the thermostat stays closed — this blocks coolant from flowing to the radiator so the engine warms up quickly. Engines are designed to operate at a specific temperature — typically 195 to 220 degrees Fahrenheit depending on the application. Operating at the correct temperature is important for fuel efficiency, emissions, and engine life.

Inside the thermostat is a small cylinder of special wax. As the coolant temperature rises and reaches the thermostat's rated temperature, the wax melts and expands. The expanding wax pushes a piston that opens the valve. Coolant flows to the radiator. As the engine cools, the wax contracts and the spring closes the valve again. Simple. Reliable. No electricity required.

A thermostat stuck in the open position allows coolant to flow to the radiator at all times — even when the engine is cold. The engine takes forever to warm up or never reaches normal operating temperature. The heater blows lukewarm air. The PCM sees low coolant temperature and sets a P0128 code — coolant temperature below thermostat regulating temperature. Fuel economy drops because the PCM keeps the fuel mixture richer for a cold engine that never warms up.

A thermostat stuck closed blocks coolant flow to the radiator. The engine temperature rises rapidly and the vehicle overheats. This is the more dangerous failure. If the temperature gauge climbs into the red zone, pull over and shut the engine off immediately. Continuing to drive an overheating engine causes head gasket failure, warped cylinder head, and possibly engine seizure.