Q: What does the thermostat do in an engine?

A: The thermostat is the brain of the cooling system. It senses and controls the temperature of the coolant. When an engine is cold, the thermostat is closed. This keeps the coolant from circulating to the radiator, allowing the engine to warm up to the most efficient operating temperature. Once the engine temperature reaches the thermostat rating, the thermostat opens. When the thermostat opens, the coolant flows to the radiator where it releases heat and circulates back to the engine.