Symptoms

• 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.
  
  

Cable

• There are two main causes of failure. The first is the failure of the thermostat cable, which sends information to the thermostat, signaling 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.
  
  

Risk

• 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.