Symptoms of a bad thermostat in cars

Updated April 17, 2017

A vehicle thermostat controls the flow of radiator coolant and/or water into an engine. A bad thermostat can cause a variety of engine-related symptoms and conditions.


A bad thermostat can cause a car to overheat if the thermostat gets stuck in a closed position. A thermostat is a heat-sensitive valve located at the end of the upper radiator hose. If a thermostat gets stuck in a closed position, radiator coolant is prevented from flowing into a car engine, resulting in overheating.


A thermostat stuck in an open position can result in underheating. Typically, a thermostat opens only when an engine reaches a certain operating temperature. If a thermostat is stuck in the open position, radiator coolant flows into an engine before it reaches normal operating temperature. This prevents adequate engine warming and results in engine underheating.

Temperature Fluctuations

A bad thermostat may open and close at random times throughout engine operation. When this occurs, an engine's temperature will fluctuate wildly as radiator coolant is prevented from flowing normally through an engine.

Housing Leaks

A vehicle thermostat is housed in a small open groove in a vehicle's manifold, where the upper radiator hose attaches. The thermostat gasket, which seals the space between a thermostat and the manifold, can leak, causing radiator fluid to leak onto the engine manifold.

Poor Engine Performance

A thermostat that is bad and/or malfunctioning can affect engine performance by drastically altering an engine's operating temperature. Engine combustion can be dramatically altered by huge fluctuations in engine operating temperature; an engine that runs too cold or too hot will not run properly.

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