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A thermostat is part of the cooling system that regulates the temperature of the coolant in the engine. The thermostat is a metal valve that has a temperature sensor built in. It can either be open or closed. When the thermostat is closed, it keeps the coolant in the engine. When the engine gets to a predetermined temperature, the thermostat opens allowing coolant to circulate. The circulation of coolant prevents the engine from overheating. If the thermostat fails to open, the car will overheat. If the thermostat fails to close, the engine may never reach the optimal temperature (~220F). The thermostat opens and closes many times when the car engine is running. It is the most common component in the cooling system to fail, causing the car to overheat.