Stuck Thermostat
A cooling system failure where the thermostat remains permanently open or closed, leading to improper engine temperatures.
A stuck thermostat is a common automotive cooling system failure where the thermostat valve becomes seized in either the fully open or fully closed position. The thermostat controls the flow of engine coolant to the radiator to regulate the engine's operating temperature.
If the thermostat is stuck closed, coolant cannot flow to the radiator to release heat, leading to rapid engine overheating and potential cylinder head warping or head gasket failure. If it is stuck open, coolant flows constantly through the radiator, preventing the engine from reaching its optimal operating temperature (typically 195 to 220 degrees Fahrenheit). This causes the engine to run cold, reducing cabin heater output, increasing fuel consumption, and causing carbon buildup.
Symptoms of a stuck closed thermostat include a rising temperature gauge, boiling coolant noises, and a cold lower radiator hose (indicating no flow). A stuck open thermostat will trigger a Check Engine Light with code P0128 (coolant temp below thermostat regulating temp). Replacing a thermostat is a straightforward repair that should always be accompanied by a fresh coolant flush.