Do I Need to Replace My Car Thermostat?

Updated on March 23, 2016

Car thermostat problems happen unexpectedly. Usually you'll start to notice that your car begins to overheat soon after you fire up the engine, or the temperature gauge on your dashboard remains below its normal temperature point.

But how do you know it is the thermostat acting up? After all, some of these same symptoms may come from a failing water pump, radiator, fan, or even a loose drive belt.

In general, when a thermostat gives out, it gets stuck in a closed or opened position. Whatever its failing state, you'll find out in a few minutes using one or more of the tests described in this guide.

But first, you need to take a quick look at how it operates so that you understand the logic behind these simple tests.


What Is a Car Thermostat?

As any other water-cooled engine on a passenger vehicle, the engine in your car operates at a temperature range of 180 F and 210 F (82 C and 99 C), approximately. To help it operate within this range, the engine uses a thermostat.

In simple terms, the thermostat responds to changes in temperature — in this case coolant temperature — by opening or closing a valve to control coolant flow between the radiator and engine. The valve itself operates through a wax-container element. Thus, the thermostat is small, simple and effective.

The expanding element container in the thermostat faces and contacts the engine coolant. As the coolant temperature rises, the fluid — or wax — in the container begins to melt and expand, pushing a small rod that separates a center plate from its surrounding mounting base to open the valve.

The Thermostat at Work

The thermostat starts at a closed position when you start the engine to help it reach operating temperature. As coolant temperature rises, it begins to open.

The opening allows hot coolant in the engine to flow into the radiator, while the water pump pushes lower-temperature coolant from the radiator into the engine. When the lower-temperature coolant reaches the thermostat container, the expanding fluid begins to contract, closing the thermostat's valve.

During engine operation, though, the thermostat actually never fully closes or opens, but gradually nears either state to control coolant flow, depending on engine operating conditions. This allow the engine to operate at the best temperature.

This ideal engine-operating temperature accomplishes several goals: It helps engine oil to lubricate efficiently and to remove harmful deposits. It reduces emissions and gas consumption, and contributes to engine performance.

Thus, the thermostat has an impact on your engine's health and longevity.

When It Goes Bad

A failed thermostat will prevent the engine from operating within its ideal temperature range and affect its performance. For example:

  • A thermostat stuck opened will cause a continuous flow of coolant, resulting in a lower operating temperature. Since the oil operates below temperature, the condition accelerates parts' wear, reducing engine efficiency and increasing emissions over time.
  • On the other hand, a thermostat stuck in the closed position will prevent coolant flow and cause the temperature to steadily rise. If you fail to notice and keep your engine running, in a matter of minutes your engine self-destructs. Literally.

Either way, your engine will suffer damage. The difference is just in the amount of time it takes.

Still, a failing thermostat is not the only cause for an abnormal engine operating temperature.

Other reasons include low coolant level, a bad water pump, a worn out or loose water pump belt, cooling system leaks, a clogged radiator, a failed radiator fan and a collapsed radiator hose.

Whatever the cause, it's a good idea to start looking into the problem before it's too late.

Common Reasons for Abnormal Engine Temperature
* Bad thermostat
* Low coolant level
* Bad water pump
* Worn-out water pump belt
* Loose water pump belt
* Cooling system leaks
* Clogged radiator
* Bad radiator fan
* Collapsed radiator hose

How Do I Know Whether My Thermostat Got Stuck Closed or Opened?

Now that you have an idea of how the thermostat operates, you can use that knowledge to investigate the problem.

  • First, open the hood and make sure the engine and radiator are cool.
  • Then, locate the thermostat. If you follow the upper radiator hose towards the engine, you will see the end of this hose connecting to the thermostat housing. Inside this housing resides the thermostat. On some vehicle models, though, the thermostat housing connects to the lower radiator hose.
  • If you need help to locate the thermostat, consult your vehicle service manual. You can buy one at your local auto parts store or online. You could also check the reference section of your public library.
  • Once you've found the thermostat, perform one of two simple tests: If you have access to the radiator cap on your car, use the following troubleshooting procedure: Checking Coolant Flow. If the radiator cap is not accessible, or you don't see the cap, go to the next troubleshooting procedure: Checking Coolant Temperature.

Some Recommendations Before You Start:

Even if your radiator cap is accessible, you can actually do both tests, since neither requires removing the thermostat from your car and they both only take a few minutes.

With an overheating engine problem, it's a good idea to have an assistant behind the wheel to shut off the engine if the temperature reaches an unsafe level while testing.

If either of your tests points to a bad thermostat, follow up with the third procedure below, How Do I Test a Car Thermostat, to test it outside of the vehicle and make sure you need to replace it.

If you are performing one of these tests because your engine is overheating, it's a good idea to have an assistant behind the wheel to shut off the engine if the temperature reaches an unsafe level while you do the test.

Checking Coolant Flow

  1. Wait for the radiator and engine to cool.
  2. Engage the parking brake and block the wheels.
  3. Remove the radiator cap, fire up the engine and let it idle.
  4. Verify that the coolant is not flowing. You can check this by looking through the radiator filler neck. The coolant shouldn't be flowing because the coolant has not reached enough temperature to cause the thermostat to open.
  5. If the coolant is not flowing, go to step 7.
  6. If you see the coolant flowing, it means the thermostat is stuck opened. And this explains your temperature gauge indicating a continuous lower-than-normal operating temperature. You need a car thermostat replacement.
  7. Wait for about 10 to 20 minutes so the engine reaches operating temperature. About this time, you'll see the coolant through the radiator filler neck beginning to flow. The coolant begins to flow at this point because it has reached a high enough temperature to cause the thermostat to open.
  8. If you don't see the coolant flowing and the temperature gauge on your dashboard is rising steadily, you have a thermostat stuck in the closed position. Shut off the engine and refer to the section, after the next procedure, How Do I Test a Car Thermostat.
  9. However, if the coolant begins to flow but the engine still overheats, you have another problem affecting the cooling system. Check the troubleshooting section of your vehicle repair manual to see other causes for engines overheating.


Checking Coolant Temperature

For this test, you'll need a needle-type kitchen thermometer. You can buy a cheap one at most pharmacies or department stores.

  1. Make sure the radiator and engine are cool, engage the parking brake and block the wheels.
  2. Start the engine and let it idle.
  3. Get a temperature reading on the engine block or cylinder head surface near the thermostat housing. Then, get a temperature reading of the upper radiator hose two to three inches away from the thermostat housing.
  4. After five minutes, read the temperature again on these same two test points and compare these readings to the previous ones.
  5. Wait for five more minutes and take another reading.
  6. At this time, you'll start to notice the temperature on the engine block or cylinder head rising while the radiator hose temperature remains about the same.

* However, if both test points remain at about the same low temperature, your car thermostat is stuck opened. This means the coolant is flowing continuously, keeping the engine from reaching operating temperature. This will confirm the continuously low temperature reading of the temperature gauge on your dashboard. Replace the thermostat.

* It takes between 15 to 20 minutes for the engine to reach operating temperature. At this time, hot coolant causes the thermostat to open. If you take another temperature reading on your two test points, you'll notice the radiator hose temperature almost as high as the engine’s temperature. This means the thermostat has opened and hot coolant is flowing through the radiator hose.

If coolant doesn't flow, you'll notice the temperature on the upper radiator hose remaining about the same, and the temperature gauge on your dashboard getting close to the red zone. This means you have a thermostat that is stuck closed.

* If necessary, troubleshoot the thermostat as described in the next section.

* If the temperature on the upper radiator hose rises, though, coolant is flowing. If your engine is still overheating, you have another problem affecting the cooling system. Check the troubleshooting section of your vehicle repair manual to see other causes that may be affecting the cooling system.


Do I Need to Replace It?

Checking your car thermostat is the best way to know if the unit has actually failed. The testing procedure doesn't require special tools or skills. But first, you need to remove the thermostat from your car.

The thermostat housing is very accessible in most cars, but you still need to follow some precautions that might apply to your particular model. If you need more help, the best way is to follow the instructions in the repair manual for your particular vehicle make and model.

Once you have removed the thermostat from your car:

  1. Visually inspect the thermostat. It should be in the closed position. If it is opened, replace it.
  2. Boil some water in a kitchen pot.
  3. When the water reaches the boiling point, grab the thermostat with a pair of pliers and submerge it in the boiling water.
  4. After a minute or less, remove the thermostat from the water.

* You should see the center section (the valve) of the unit separated from its base; the thermostat has opened. If it remains closed, you need a car thermostat replacement. This is what's causing your engine to overheat.

* However, if the thermostat has opened and you're investigating why the engine is overheating, the cause of the problem lies somewhere else. Have you changed the coolant at the recommended manufacturer schedule? Your coolant may be too old to do its job anymore. Also, as pointed before, look for low coolant level, cooling system leaks, a bad water pump or radiator fan, a loose or worn out water pump belt.

Car thermostat problems can be diagnosed with these simple troubleshooting procedures. They only take a few minutes and will help you find the cause of an unusual engine operating temperature.

Even more, they'll help you save your engine — and your wallet — from damage. If your troubleshooting procedures tells you the thermostat is working, find the cause of the problem as soon as possible. In this case, the vehicle repair manual for your particular car make and model will be of great help.


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    • Dan Ferrell profile image

      Dan Ferrell 2 months ago

      Hi Maribe,

      There're different scenarios here.

      Make sure you bought the correct thermostat for your make and model.

      Does your car overheat only at idle, while in town - how about at highway speeds? If it cools down while driving at highway speeds than you may have a radiator air flow issue. Make sure there no debris covering the front of the radiator.

      Did you replace the coolant recently, or when replacing the thermostat? There may be air pockets in the cooling system if the engine overheats at any speed - than you need to purge the system.

      Have the radiator checked as well, it may be clogging, preventing proper coolant flow.

      Less common issues, but happen: Do you know the engine is actually overheating?

      Does the engine, hoses, radiator (top, bottom) feel hot when gauge goes to red? Maybe the temp sender is not functioning properly. Was it disconnected by accident while replacing the thermostat?

      Another more serious issue would be a torn-leaking head gasket. Have the engine block tested at an auto shop.

      Good luck

    • profile image

      Maribe 2 months ago

      My car's engine over heating. I took out a thermostat and travel without it and the engine was cool. I bought a new thermostat and put it in still the engine over heating. Please help.

    • Dan Ferrell profile image

      Dan Ferrell 8 months ago

      Hi Tammy,

      Take a look at this guide, it tells you how to check the thermostat.

      Heat not coming out doesn't necessarily means a bad thermostat. The core may be clogged.

      Good luck.

    • Dan Ferrell profile image

      Dan Ferrell 8 months ago

      Hi Tammy,

      A simple way it to check the upper radiator hose. If you touch the upper radiator upon starting the engine, it feel cool. After a few minutes of idling (perhaps 15 to 20 minutes), you'll notice the same hose getting hot because the thermostat has opened and letting hot coolant into the radiator.

      Whit a stuck open thermostat coolant wont get a chance to get hot. A stuck closed thermostat will cause the engine to overheat.

      But a bad thermostat is not the only cause for a no heat condition. The heater core might be clogged. Check the thermostat to make sure it's working or not working.

      Good Luck.

    • profile image

      Tammy 8 months ago

      How do you know if you need a new theromostat if heat does not come out??

    • everythingbike profile image

      Peter 2 years ago from The Road with corners.

      Great article and perfect for the machincally challenged. As a motor mechanic I always use the "touch incoming and outgoing hoses" test. Works 90% of the time. Thermostats are quite easily and cheaply replaced so rather change them if there is doubt. Thumbs up on this article.

    • favored profile image

      Fay Favored 2 years ago from USA

      Good article. Sharing it as well. Thumbs up & Pinned. Thanks to Aladdins Cave for tweeting it.

    • Aladdins Cave profile image

      Aladdins Cave 2 years ago from Melbourne, Australia

      Well done, I hope lots of people read this.

      Could save them a lot of money before they blow a head gasket.

      Thumbs up and I've tweeted it for you as well

      Cheers from DOWNUNDER


      In my book "From Australia to Germany" I have written about how our 4x4 turned into a steam engine in Afghanistan because of cooling problem. Just saying

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