A car thermostat is a valve that regulates the flow of coolant through the engine. When the engine is cold, the thermostat is closed to allow the engine to warm up quickly. As the engine reaches operating temperature, the thermostat opens to allow coolant to flow through the radiator, where it is cooled. If the thermostat becomes stuck closed, it can lead to a number of problems.
One of the most common problems that can occur when a thermostat is stuck closed is overheating. When the thermostat is closed, coolant cannot circulate through the radiator, which can cause the engine to overheat. Overheating can damage the engine and lead to costly repairs.