Why Thermostats Fail

Updated November 21, 2016

The thermostat in your automobile is responsible for controlling the movement of coolant through the engine. It is designed to assist the engine to rapidly reach and consistently maintain the ideal running temperature. There are several reasons why the thermostat may fail to operate properly.


When the thermostat fails, it causes the car to run at an inappropriate temperature. Depending on how it fails, it can cause the car to overheat, or under heat, both of which can do permanent and costly damage to the engine if not promptly diagnosed and repaired.


There are two main causes of failure. The first is the failure of the thermostat cable, which sends information to the thermostat, signalling it to open and close. The cable typically breaks at the point where the cable enters the connector. This cable can be quickly removed and replaced, if it is the cause of failure.

Stuck Closed

The most common cause of failure is the thermostat's inability to open, and remain open long enough to cool the engine. When the thermostat sticks closed, it causes the car to overheat. Correcting the issue will typically require replacement.

Stuck Open

A thermostat, in some cases, may stick open. This will cause the engine to receive too much coolant and can cause other engine systems to fail. When this occurs, the thremostat should be promptly replaced by a professional.


The risk of failing is significantly higher if an aftermarket thermostat is put on the engine. Using a thermostat that is manufactured by a car dealer may prevent additional replacement and repair.

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About the Author

Misty Barton has been working in the fields of composition and journalism for over 10 years. She has a Bachelor of Science in English education and a Master of Arts in English and composition. She has written for various online publications including a blog that specifically addresses the concerns of work-at-home mothers.