Signs that a thermostat is bad

Written by jen davis
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Signs that a thermostat is bad
Thermostats can be damaged by other cooling system issues, so if you recently had cooling system problems, check to make sure your thermostat is still working. (red car 2 image by Viachaslau Makouski from

A bad thermostat can mean serious problems for your car. If your engine gets too hot or runs too cold for too long a time, it can permanently damage your vehicle and cost you hundreds of dollars in repairs. There are several signs and symptoms you should look for if you think your vehicle's thermostat is not functioning correctly.


In the event your vehicle's thermostat becomes stuck in the closed position, it will not allow coolant to flow through the motor, and your engine will become hotter than usual. The hotter your engine gets, the more likely it is to suffer a major mechanical problem, such as a blown head gasket. If your vehicle is overheating, the thermostat might be to blame, especially if you have enough coolant and cannot find a problem with the radiator or water pump.

Running Too Cool

An engine that runs at a cooler temperature than what is recommended often does not attract the attention that an overheating motor will. However, running too cool will keep your vehicle from performing its best and prevent your heater from working properly in cold weather.

Emissions Problems

The temperature of your engine affects the way fuel and oil are consumed and used by the engine. If your engine is running too cool, the fuel might condense on the cylinders and decrease your fuel mileage. It might also cause your vehicle to run lean, which will trigger your O2 sensors and affect your vehicle's emissions. A vehicle running too hot might consume too much fuel or burn oil, which will also cause emissions issues. If you notice too much exhaust, coloured exhaust or your car fails an emissions test, the thermostat might be to blame.

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