How To Test A Car Thermostat?

Are you tired of feeling helpless when your car’s temperature gauge starts creeping into the red zone? Engine overheating can be a nerve-wracking experience, especially if you’re unsure how to diagnose the problem.

One component that often comes under suspicion is the car thermostat. If you’re scratching your head wondering, “How on earth do I test a car thermostat?

Fortunately though, you’ve stumbled upon this comprehensive guide that will assist you and provide you with the necessary information on how to test a car thermostat.

Testing a car thermostat is relatively easy with the right tools and knowledge.

The whole purpose of a car thermostat is to regulate and maintain the engine’s optimal operating temperature, ensuring efficient combustion, optimal fuel consumption, and protection against overheating of the engine.

Before we dive into the testing part, it’s important that you know the basics on what does a car thermostat do.

A car thermostat uses a heat-sensitive element to regulate coolant flow. When the engine is cold, the thermostat remains closed, directing coolant back to the radiator.

As the engine warms up, the thermostat gradually opens, allowing coolant to circulate and maintain the desired operating temperature. This continuous regulation ensures efficient engine performance

Here’s How to Test a Car Thermostat.

how to test a car thermostat

With just a few minutes and some basic tools, you can easily perform this test. So, let’s dive in!

Step 1: Gather the Essentials

To begin, gather the necessary tools and materials. You’ll need:

  1. A bucket or container to hold hot water.
  2. Hot water (boiling water is ideal).
  3. A thermometer (optional but useful for temperature monitoring).

Step 2: Understand the Thermostat

Before proceeding, it’s helpful to familiarize yourself with the thermostat’s role. The thermostat regulates the engine’s temperature by controlling the flow of coolant between the radiator and the engine block.

When the thermostat reaches its designated opening temperature, it allows coolant to flow, preventing overheating. Most thermostats are marked with the degree at which they should open.

Step 3: Test the Thermostat

Now, let’s test the thermostat. Follow these steps:

  • Identify the thermostat location in your car. To locate the car thermostat, find the lower radiator hose that goes straight to the engine. Following the hose you will find the thermostat buried underneath.

how to test a car thermostat

  • Place a container or bucket beneath the thermostat housing to catch any water.


  • Pour hot water over the thermostat. Ensure the water temperature matches the thermostat’s designated opening temperature. For example, if your thermostat is rated at 180 degrees Fahrenheit, use water at or near this temperature.


  • Observe the thermostat’s behavior. Watch for any signs of movement or opening.
    • If the thermostat opens up fully, allowing water to flow, and closes smoothly afterward, it is functioning correctly. In this case, the thermostat is not the cause of your overheating issue.
    • However, if the thermostat fails to open or only partially opens, it may be stuck and require further attention or replacement.

Reasons Why a Car Thermostat Goes Bad

Normal operating temperature should be 195 to 225 degrees Fahrenheit, so when it comes to symptoms of a bad thermostat it can go in two different directions.

Stuck open thermostat: With this symptom you might be experiencing the engine light problem. A code known for this cause is p0128 which means this coolant is taking normal than usual to reach operating temperature.

The OBDII scanner we used to determine this issue is a great tool to save you money. Your overall vehicle maintenance is at your own finger tips, it allows you to reset the computer after repairs have been made and much more!

The second symptom could be the temperature gauge taking longer than usual to reach the halfway mark. A stuck open thermostat allows coolant to travel from the engine through the radiator which cools down the coolant which goes back into the engine.

Since the ‘coolant process’ is a never ending cycle, it delays the time of the time for the engine to reach the operating temperature.

Lack of heat: The other symptom would be a poor heating performance in the interior of the car. Since the coolant is always getting cooled by the radiator, this will affect heat in your car because hot coolant is the contributor to your car’s heating system.

If a car thermostat becomes stuck in the closed position, it can prevent coolant circulation and lead to overheating, potentially causing engine damage.

Reasons why a car thermostat may get stuck:

  • Over time, sediment and debris can accumulate in the cooling system, affecting the thermostat’s movement and causing it to stick in a closed position.


  • Rust and corrosion can develop on the thermostat, impeding its movement. This can occur due to old coolant or moisture in the cooling system.


  • The thermostat may experience internal mechanical failures, such as a broken spring or valve, causing it to become stuck.

Temp gauge on hot: A stuck closed thermostat can be the cause of your temperature gauge on the hot position which means your engine is overheating.

Since the thermostat never opens, it will never be able to go through the radiator which cools down the coolant which goes back to the engine.

Knowing how to unstick a thermostat in a car is imperative to restore its functionality and prevent engine overheating.

Leaking coolant: This symptom occurs when you have a closed thermostat. The coolant could be leaking from the thermostat housing or it could leak through one of the hoses.

The reason why this happens is because when the heat increases, the pressure also increases and it causes the objects to expand until they pop in which case it results in a leak.

Final Thoughts

Congratulations! You’ve successfully tested your car thermostat. By performing this simple procedure, you’ve gained valuable insights into its functionality and determined whether it contributes to your overheating problem.

Remember, if your thermostat appears to be malfunctioning, it’s crucial to address the issue promptly to avoid potential engine damage. If you’re unsure or uncomfortable with the process, consulting a qualified mechanic is always a wise decision.

We hope this step-by-step guide has been informative and helpful. Testing your car thermostat is an essential diagnostic tool that can save you time, money, and frustration.

Don’t let overheating issues drive you crazy—take control and ensure your engine stays cool and reliable on every journey!

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