How to Tell a Thermostat Needs Replacing in a Car

by Zyon Silket

Diagnosing a thermostat failure in an automobile can be a challenge because a thermostat can fail in the open position or the closed position and both issues present different symptoms. Those symptoms also may not always exclude other components of the cooling system. Ruling out other mechanical failures will make it easier to pinpoint the thermostat as an issue. The great thing about troubleshooting the cooling system is that it does not require tools but it may require access to a mechanic who can test a check-engine light.

No Heat Inside of the Cabin

Inspect the front passenger-side floor for any signs of antifreeze. Antifreeze is yellow, orange or green and has a sweet smell. If you locate antifreeze on the floor, the heater core has malfunctioned and is leaking. The thermostat will not cause this to happen. Replace the heater core.

Inspect the blower motor by turning the engine on. With the engine on, turn the heater fan on full blast. Inspect the vents to see if air blows out. If air does not blow out, replace the blower motor. The thermostat will not prevent the blower motor from working.

Inspect the inside windshield for a greasy film. Run your finger across the glass where the defrost blower would push air onto the windshield. If a greasy film is present on the windshield, the heater core is clogged. Flush the heater core or replace it. The thermostat will not cause this.

Replace the thermostat if the inspection of the passenger-side floorboard, blower motor and the inspection of the inside windshield does not yield positive results. The thermostat regulates the temperature of the antifreeze that flows into the heater core. If the thermostat fails in the open position, antifreeze will flow though the heater core unrestricted and the heater will not produce heat.

Engine Overheats

Start the car and allow the temperature gauge to reach half-way. Inspect for a check-engine light. If a check-engine light appears, take the vehicle to a certified repair facility and have them diagnose the check-engine light. If the code indicates that the thermostat has failed to work, replace the thermostat. It is possible to purchase a code reader from many popular automobile-parts retailers and diagnose the code if you choose to do so. However, unless you plan to use a code reader on a regular basis the cost may be prohibitive.

Check the radiator for fluid levels. The easiest way to do this is to check the radiator overflow tank. If the tank is half-way full, the radiator is full and will cool the engine as normal. If the overflow tank is empty, inspect the radiator and fill as needed. With the radiator filled, run the engine and inspect for overheating.

Remove the thermostat from the thermostat housing and then bolt the thermostat housing back into place. Start the engine and inspect it for overheating. If the engine does not overheat with the thermostat removed, replace the thermostat. If the engine does overheat, the issue is not with the thermostat. Inspect the thermostat-relay switch.

Flush the coolant system. The engine has small passageways inside of it that antifreeze flows though to keep the intake and the engine block cooled. These passageways can clog, preventing fluid from passing through the engine. After flushing the engine, inspect for overheating.


  • check If the thermostat fails in the open position and the inside of the vehicle does not produce heat, do not expect engine overheating. Just as the antifreeze flows unrestricted through the heater core, it also flows unrestricted through the engine. This prevents the engine from overheating.

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About the Author

Since 2006 Zyon Silket has been writing for companies such as SEOWhat, L&C Freelancing and T-Mobile Wireless. He has extensive experience working in supervisory roles within the wireless and Internet technologies fields. Silket is pursuing a Bachelor of Arts in business management and network technologies at Lehigh Carbon Community College.

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  • photo_camera radiator humor image by John Sfondilias from