What Happens When a Thermostat Is Stuck?

Written by kevin fobbs
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What Happens When a Thermostat Is Stuck?
A stuck car thermostat can cause the engine to overheat. (Girl repair car on road image by Vasyl Dudenko from Fotolia.com)

When a thermostat is stuck there can be two problems. First, if the thermostat is stuck open, then the car will receive no heat and will remain cold; if it is stuck closed, then the car will overheat and seriously damage the engine, rendering it inoperable.

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Thermostat Function

The main function of your vehicle's thermostat is to control the flow of the coolant to the engine. When your vehicle's engine has reached its correct operating temperature, the thermostat opens, allowing the flowing coolant to absorb the heat from the engine. The vehicle coolant directs the heat away from the engine to the radiator to cool off.


Your vehicle's thermostat can become defective for several reasons. For instance, if your radiator cap is defective, the coolant level is low because of a leak, or there is inadequate airflow circulating through your radiator, the thermostat will become seriously compromised and lose the ability to assist in absorbing and dissipating the engine's heat.

Test Thermostat

Test your thermostat to see if it is damaged. Start your vehicle's engine and touch the upper radiator hose. If the hose does not feel hot after the engine has warmed up, it means the vehicle's thermostat is not opening and should be replaced. Also, consider removing the thermostat and dipping it into a pan of boiling water. It should open if not damaged.

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