Troubleshooting a Stuck Radiator Thermostat

hand adjusting car thermostat device

Vehicle engine cooling systems consist of a radiator, engine water jacket, assorted connecting hoses, fan, thermostat, water pump, and a coolant mix. The system is filled with coolant and circulated through the components with a water pump.

The thermostat is placed in a position to retard coolant flow when the engine is cool, allowing it to warm to operating temperature.

When that temperature is reached, the thermostat opens, allowing the water pump to move the coolant through the radiator. The fan pulls air through the radiator to lower the temperature of the coolant, which is then pumped through the engine.

This cycle is continuous and maintains the engine at the best operating temperature. If the thermostat fails to do its job, you then have temperature related problems.


person in jacket in car

If your engine temperature is staying too cool or too hot when operating your vehicle, the problem most likely is the thermostat. Thermostats actually do wear out eventually and fail to do their intended job of controlling a precise temperature range of your vehicle engine.

Thermostats fail by becoming stuck in either the open stage (resulting in too cool of engine temperature) or in the closed stage (resulting in an overheated engine). An overheated engine is a dangerous problem, as it can cause engine damage very quickly.

Engine components will warp, crack, and parts that move will lose their lubrication, causing total engine failure. The too cool engine will suffer damage, but it is slower to happen and not as disastrous.

A stuck open thermostat will not provide enough heat to properly operate the vehicle heater. Therefore an inoperable heater is a symptom of a stuck thermostat.


Check your coolant level when the engine is cool and top it off if needed before proceeding. Low coolant levels can mimic thermostat problems. If after topping off coolant you still experience a too cool or a too hot engine temperature, inspect the thermostat.

Remove and Replace

hands holding car thermostat

Thermostats are usually located in a bolt on housing at top of the engine where the top radiator hose is connected.

Replacement requires removal of the housing, cleaning the housing, and installing a new thermostat. Due to the low cost of thermostats, attempts to repair them are not economically justifiable.

Sometimes the design engineers see fit to place the thermostat in the radiator hose, and the repair is to replace the complete hose assembly.

Once you locate the thermostat, allow the engine to cool for a couple of hours before attempting a repair. The radiator coolant can cause severe burns to your body.

Drain the radiator into a suitable container and follow instructions in the service manual for your specific vehicle for thermostat replacement.

A stuck open thermostat will show the problem on removal and visual inspection. A closed thermostat can be checked by placing it in a saucepan of water and heating the water to boiling. If the thermostat remains closed in the boiling water, then it should be replaced.

The drained out coolant can be filtered through cloth and put back in radiator, although a thermostat replacement is a good time to do a cooling system flush and refill with new coolant.

A mix of 50% antifreeze and 50% distilled water is the preferred mix for radiator coolant for most vehicles. Use of distilled water is recommended because of the galvanic corrosive action in modern engines made from different metals.

Dissimilar metals will accelerate corrosion to the extreme of causing corrosion holes in engine components and loss of coolant through those corrosion holes.

It is good economics to spend a little extra money for distilled water for your coolant mix rather than having corrosion holes in engine components.