A car thermostat accelerates car engine warm up and regulates its operating temperature. It maintains the minimum required operating temperature in an engine by regulating the amount of coolant sent to the radiator for cooling.
A car thermostat closes when an engine starts cold and circulates the coolant within the car engine until it warms up. As the engine reaches optimum operating temperature, the thermostat opens and causes the coolant to flow out of one radiator hose through the radiator, and back again to the engine through the other radiator hose. The primary purpose of a car radiator is to remove heat from the coolant so that the engine does not overheat.