How Coolant-Regulating Thermostats Fail (And What You Should Do About It)

How Coolant-Regulating Thermostats Fail (And What You Should Do About It)

11th Mar 2019

Located between the radiator and engine, the thermostat plays an important role in regulating the flow of coolant. During startup, the thermostat will remain closed, thereby restricting the flow of coolant to your car's engine. As the coolant heats up, however, the thermostat will open (usually around 200 degrees Fahrenheit), allowing coolant to flow into and through the engine. But there are times when thermostats fail, some of which may include the following.

Stuck Open

The most common way in which thermostats fail is that they stick open. As previously mentioned, thermostats are designed to open and close depending on the temperature of the coolant. When the coolant heats up to the appropriate temperature, the thermostat opens. And when the coolant cools back down -- after the engine has been turned off and left to sit for a few hours -- the thermostat closes. Unfortunately, some thermostats may stick open, meaning they allow both cool and hot coolant to flow through them.

If your car's thermostat is stuck open, the engine won't be able to warm up. Therefore, you can diagnose this problem by monitoring the temperature gauge on your car's dashboard. If it remains at the very bottom, suggesting the engine is cool, after idling or driving for five or 10 minutes, the thermostat is probably stuck open.

Stuck Closed

While most thermostats fail by sticking open, some fail by sticking closed. If your car's thermostat is stuck closed, neither cool nor hot coolant will be able to flow through. And without coolant flow through the radiator, into the engine and back through the radiator, your car will quickly overheat.

You can usually tell if your thermostat is stuck closed by monitoring the engine temperature gauge on your car's dashboard. If you see the needle rising past the halfway mark or into the red zone, it means your engine overheating, which could be the result of a thermostat that's stuck closed.

Jiggle Pin Clogged

A lesser-known way in which thermostats fail is the jiggle pin becomes clogged. Assuming your thermostat has a jiggle pin, it could become clogged with gasket material or debris. With the jiggle pin clogged, air may become trapped between your car's radiator and the engine. The jiggle pin prevents this from happening by allowing air to push its way past the thermostat. If the jiggle pin is clogged, though, it won't be able to vent trapped air, which could result in an inefficient cooling system.

How Often Do Thermostats Fail?

Consisting of just metal, rubber and a large spring, thermostats are relatively simple in design, so they aren't prone to premature failure. Nonetheless, thermostats can and do fail. Some thermostats last longer than others, but you can typically expect about 10 years or 100,000 miles out of a thermostats before it fails.