How to Check your Car's Thermostat

The thermostat in your car is responsible for regulating the flow of coolant for your vehicle’s engine. It responds to temperature changes, by a small open or closed valve. For example, when you start your car in the morning, the thermostat is in a closed position. Your car thermostat is designed to open up once your car’s engine reaches its full operating temperature; allowing coolant to flow into the radiator. This helps prevent the engine from overheating. When you have a faulty thermostat it can damage your engine and other mechanisms of the cooling system won’t be able to do their job. To make sure your car thermostat is in working condition, it’s important to check it. Follow the steps below, to learn how to check your car thermostat.

1. Have your car on level ground and make sure the engine and radiator are cool.
2. Open the car hood and find the thermostat. You can do this by following the upper radiator hose towards the engine. The end of this hose will be attached to the thermostat housing. With most cars, you should be able to find your thermostat in the housing. There are some vehicles where the thermostat housing will be attached to the lower radiator hose. If you need additional guidance locating your thermostat, review your vehicle service manual.
3. Next, you will need to test your car thermostat. You can do this by removing the radiator cap on your car to check the coolant flow. Start your car’s engine and allow it to idle. Look through the radiator filler neck to see if the coolant flows. At this time, it should not be flowing as your car has not reached the operating temperature to cause the thermostat to open. 
4. If you find the coolant is flowing, it means the thermostat valve is open. This is usually a good indicator of if you need to replace your car thermostat.
5. If you notice the coolant is not flowing, wait until your car’s engine has reached the operating temperature. Have another look through the radiator filler neck to see if the coolant has begun to flow. If you still don’t see the coolant flowing and your car’s temperature gauge starts to rise, this means your car’s thermostat is in a closed position.

1. Have your car on level ground and make sure the engine and radiator are cool.
2. Start your car’s engine and allow it to idle.
3. Use a cooking thermometer or a hydrometer to obtain a temperature reading from the engine block (also known as the cylinder block) and the upper radiator hose. The end of this hose will be attached to the thermostat housing.
4. Wait for approximately 10 minutes and perform another reading on both parts. Compare these results with the previous ones.
5. Wait another 10 minutes and take another reading on both parts again. You should notice the increase of temperature from the engine block while the upper radiator hose temperature should remain the same. If the engine block temperature does not change, it means your car thermostat is stuck open. This means you will most likely need to replace your car’s thermostat as the engine cannot reach a specific temperature.