Skip to content
Home » How to Test a Car Thermostat?

How to Test a Car Thermostat?

How to Test a Car Thermostat Featured Image

As time goes on, cars are getting more and more reliable than ever before. But things can go wrong from time to time. One of the most common issues cars face today is a faulty thermostat since these devices get exposed to large differences in temperature.

But even when you suspect something might be wrong with it, determining if your car thermostat is busted is a different issue.

This article discusses how to diagnose a faulty thermostat and test one. So read on till the end.

What is a Car Thermostat?

A car thermostat is a crucial component of a car. It plays an important part in maintaining your car’s temperature by controlling the coolant flow into the radiator.

If the engine is off and is cold, the thermostat is set in a closed phase, but when the engine runs and reaches a certain temperature, the valve opens up. Once open, the thermostat allows the coolant to circulate to and from the radiator, cooling the engine. This circulation allows the engine to be at an optimal temperature when running.

If the thermostat becomes faulty, the coolant can’t circulate effectively. Sometimes the thermostat gets stuck in either the open or closed phase.

If the thermostat remains open, it will stop the engine from reaching its optimal temperature. On the other hand, if it is stuck in the closed position, there will be no flow of coolant into the radiator leading to your engine overheating.

Both these instances can spell bad for your engine if not addressed immediately. You must get your thermostat replaced if it shows symptoms of malfunctioning.

What are the Symptoms of a Bad Thermostat

What are the Symptoms of a Bad Thermostat

Below are a few symptoms that can usually be seen when your car thermostat starts malfunctioning:

The temperature gauge reads overheated

A temperature gauge that shows the car to be overheated is usually the first sign of something wrong with your thermostat.

Erratic temperature readings

Right after starting your car, you will usually see the temperature gauge signally cold. While driving, the gauge will slowly rise to the middle of the indicator, which is the optimal engine temperature.

However, if your thermostat is faulty, the gauge will signal several different levels while driving. This is especially dangerous if it is stuck in the hot position for too long, as it could damage your engine.

Another thing to notice is whether the temperature gauge never reaches the mid-point while driving. If you see this happening, you should turn on the heater. If there is no hot air blowing in, it means your thermostat is broken.

Read Also: How Long Does It Take to Replace a Car Battery

Leaking thermostat

If you find that your thermostat is leaking coolant, it is a sign that it has become faulty. You will notice this leak around the thermostat’s housing.

Heater malfunction

If you have a faulty thermostat and are driving during winter or in a place with a cooler climate, you will find that the heater is not functioning properly. This may be due to the thermostat being stuck in the open phase, which means the coolant will be circulating the radiator even when not required.

Varying air temperatures

If you suddenly find your car’s climate control not working properly, for example, having abrupt temperature changes, it is a sign that your thermostat is faulty. Any shift in air temperature compared to the setting you have turned on indicates something is wrong with the thermostat.

Unusual sounds

If you hear weird noises from the engine, radiator, or both, it signifies something is wrong with your thermostat. The noise can sound like boiling water, gurgling, or even a knocking.

What Can Cause a Car Thermostat to Malfunction?

There are a bunch of things that can cause your thermostat to behave erratically. One of the most common reasons for a malfunctioning is a dirty thermostat. Dust build-up on a thermostat can cause the sensors to malfunction.

You can also have a wire loose from your thermostat. This can also cause problems as a disrupted signal will not allow your thermostat to work correctly.

Sometimes the position where the thermostat is located can be an issue. Thermostats are best suited to be attached in a place where the temperature is stable. Using them in a location where the temperature varies may cause the thermostat to malfunction.

How to Test a Car Thermostat

How to Test a Car Thermostat

You can test a car’s thermostat by removing or keeping it in two main ways. Both have their pros and cons.

How to test a car thermostat without removing it

Although removing the thermostat is a surefire way of checking a car thermostat, you can also test it without removing it from the car.

Here is a guide on how to do so:

  1. Keep your car on ground level and ensure the engine and radiator are cool.
  2. Open the hood and follow the upper radiator hose towards the engine. The thermostat should be in the housing connected to the hose. This may be in another location in different cars, so you should read your car’s manual if you can’t find the thermostat.
  3. Now, to check the thermostat, remove the radiator cap and check the flow of the coolant. Since your car is cool, the coolant should not be flowing.
  4. If you see that the coolant is flowing, the thermostat is stuck in the open position, which means the thermostat needs to be replaced.
  5. However, if you find the coolant is not flowing, it doesn’t automatically mean your thermostat is fine. Start your engine and wait until it has reached operating temperature, then open the radiator cap and check the coolant flow. If you still don’t see any flow and your car’s temperature keeps rising, the thermostat is stuck in the closed position and needs replacement.

How to test a car thermostat by removing it

Here we will outline how to test your car’s thermostat by removing it from the vehicle. This is the best way to determine a thermostat’s state, but if you don’t feel confident about your mechanical skills, you can use the other method mentioned above.

  1. First, inspect the thermostat after removing it. If it is in the open position at room temperature, the thermostat is broken and needs to be replaced.
  2. If the thermostat is closed, fill a bowl with water and heat it to 20 degrees above the opening temperature of the thermostat. You can do so by using a regular thermometer.
  3. Gently lower the thermostat into the heated water and keep an eye on the temperature using the other thermometer, ensuring the water remains at least 20 degrees above the rated limit of the thermostat.
  4. If the thermostat opens up, it is fine. If it does not, even after repeatedly being exposed to high temperatures, it is broken.
  5. Now leave the thermostat in the bowl and let the water cool. If the thermostat closes after the water temperature is below, it works fine.


A faulty thermostat is a common car issue faced by people regularly. If you find your car’s temperature gauge behaving erratically and your car’s climate control not working as it should, your car’s thermostat is faulty.

However, your broken thermostat doesn’t mean you must take your car to the garage. Just follow the guide provided in the article to diagnose the thermostat, and you can order a new thermostat and fix it yourself.


How do you check a car thermostat?

You can check the thermostat by checking the flow of the coolant or by removing the thermostat and seeing if it’s open at room temperature.

How do I know my car thermostat is bad?

If your gauge fluctuates or your coolant leaks, it may signal that your thermostat needs to be replaced.

Can you test a car thermostat in boiling water?

Yes, you can dunk a thermostat under boiling water, and if you see the valve is still closed, it means the thermostat is faulty.

How do you know if your thermostat is stuck closed?

The valves are probably stuck closed if there is no coolant flow, or the thermostat valve is closed even when submerged in boiling water.

Editorial Choices for You