Thermostat

When the engine is cold, the thermostat stays closed so the engine warms up quickly. Once it reaches operating temperature, it opens and lets coolant flow through the radiator. Some modern engines use electrically heated (map-controlled) thermostats so the control unit can vary engine temperature with load.

Most thermostats use a wax element that expands when warm. Some engines use electronically controlled thermostats so the operating temperature can vary with load. Engines are most efficient and wear least at their designed operating temperature, typically around 90 °C for coolant. Some cars use a map-controlled thermostat that runs the engine hotter at part load for economy and cooler at full load for protection. The thermostat housing is a common place for small coolant leaks.

A wax capsule expands as it heats up and pushes the valve open at a set temperature. On map-controlled thermostats, a heater can open it earlier. When the engine is cold, the thermostat stays closed so coolant only circulates through the engine. As temperature rises, it opens and sends coolant through the radiator.

A thermostat stuck open causes slow warm-up, poor heating and code P0128. One stuck closed causes overheating, which is far more dangerous. A thermostat stuck open causes slow warm-up, weak heating and higher consumption; one stuck closed causes overheating. Replace it with one of the correct opening temperature.