A thermostat regulates engine coolant temperature by opening and closing to control coolant flow.
When an engine is cold, the thermostat remains mostly closed, directing coolant through the engine rather than the radiator. This allows the engine to warm up quickly and reach its designed operating temperature. As the coolant heats, the thermostat opens progressively and sends more coolant through the radiator, where heat is released to the surrounding air.
Most modern automotive thermostats use a sealed wax pellet. Heat causes the wax to melt and expand, pushing a rod that opens the valve. The thermostat continually adjusts its opening as engine load, vehicle speed, and outside temperature change, rather than simply switching once from closed to open.
The fan and radiator also matter, but neither is the primary temperature-regulating valve. The radiator removes heat, while the fan moves air through it, especially when the vehicle is moving slowly. A thermostat stuck open can make warm-up slow and reduce heater performance; one stuck closed can cause rapid overheating. Typical thermostat ratings are roughly 70–90°C (160–200°F).