The thermostat cuts off the flow of coolant to the radiator when the engine is cold so that heat generated by the engine warms the coolant. Once the coolant warms to a specific level, a valve in the thermostat is supposed to open so that coolant flows through the radiator to cool off before being pumped back through the engine in a continuous cycle. If a thermostat is open all the time, the coolant will constantly be circulating through the radiator and the engine will take longer to warm up as a result. If a thermostat is stuck in the closed position, the engine could overheat because coolant isn’t circulating through the radiator and shedding some heat. In either case, the thermostat should be replaced. Even if the thermostat opens and closes when it’s supposed to, the housing gasket could develop a leak and need replacing. The thermostat is typically located where the upper radiator hose enters the engine. Thermostat replacement usually isn’t listed in vehicle maintenance schedules, and some mechanics advise that as long as it’s working there’s no need to replace it. On the other hand, if the coolant is being flushed at, say, 100,000 miles, or radiator hoses are being replaced, many repair shops will suggest replacing the thermostat at the same time.
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