How To Tell If Thermostat Is Stuck Closed? new for 2022

When your thermostat isn’t working properly, you may be wondering how to tell if it is stuck open. Depending on the model, there are a few different ways to determine if your thermostat is stuck open.

This post will answer your question about “how to tell if thermostat is stuck open?” and provide relevant information.

how to tell if thermostat is stuck open
how to tell if thermostat is stuck open


What Is the Purpose of the Engine Thermostat?

What Is the Purpose of the Engine Thermostat?
What Is the Purpose of the Engine Thermostat?

A car’s engine can be damaged if it is too hot, as the metal will warp. However, when the engine is cold, it runs more efficiently and so keeping the temperature at a right level is important with regards to the thermostat.

The coolant does what you expect—it cools the engine down. The engine thermostat can open and close. When it’s open, the coolant flows through, lowering the temperature of the engine. When it’s closed, the coolant is blocked until the engine warms up. Engine manufacturers will install a thermostat that opens at operating temperature (usually either 180º F or 212° F). While thermostat controls flow ofcoolant, radiator’s job is tocool fluid

  • When the engine is cold, the radiator fluid is cold, so the thermostat is in the closed position. This prevents coolant from flowing outside of the engine and into a heated area such as a car’s interior.
  • When the engine starts up, heat flows from the engine to the coolant. Once the coolant reaches a specific temperature, a thermostat opens and allows it to circulate through to the radiator.

Possible Symptoms of a Faulty Thermostat

Possible Symptoms of a Faulty Thermostat
Possible Symptoms of a Faulty Thermostat
  • A car’s overheating can be caused by a stuck thermostat, which is one of the most common reasons cars overheat. Radiator fluid can also steam or bubble, indicating that the system isn’t operating as it should.
  • The engine’s running could not have heated up the room enough to make it comfortable, suggesting that there may be a problem with the heater.
  • One of the warning lights on your dashboard may be on because there is a problem with your engine. This could mean that you need to take your car in for service.
  • The temperature gauge remains on “cold” all the time.

Will a new thermostat solve my problems?

Will a new thermostat solve my problems?
Will a new thermostat solve my problems?

If the thermostat does not work and is less than a year old, it is likely that the device is faulty and needs to be replaced.

How to tell if thermostat is stuck open? 3 Ways to know

How to tell if thermostat is stuck open? 3 Ways to know
How to tell if thermostat is stuck open? 3 Ways to know

When the thermostat stops working, there are several indicators that can help you determine the cause. See below for more information.

Observing the Coolant Flow

1. Take the cap off of your radiator.

Locate the radiator located behind the grill of your vehicle. Look for a circular metal cap on either side and turn it counterclockwise to loosen it.

  • If you have recently driven a car, wait until the engine has cooled completely before starting it. This will ensure the test is accurate.
  • Do not open the radiator cap immediately after running your vehicle since it could be very hot and cause burns.

2. Start your engine and let it idle for 10-20 minutes.

When you first start your vehicle, the thermostat will stay closed and there won’t be any coolant flowing into the radiator. Leave the engine running for about 10-20 minutes so it can reach its optimal operating temperature.

  • If you notice coolant flowing into the radiator right when you start your vehicle, then the thermostat is stuck in the open position.
  • Don’t run your vehicle in an enclosed space since it will create harmful fumes.

3. Look into your radiator to check if coolant is flowing through it.

If the engine has been running for at least 10-20 minutes and you see coolant flowing through the radiator, then it is likely that the thermostat opened and is working properly. If not, then it may be stuck closed or blocked.

  • If you cannot determine whether or not the coolant is flowing in the radiator, shine a flashlight inside so you can see it better.

4. Check if the engine temperature gauge enters the red danger zone.

If the engine temperature is in the red section of your dashboard gauge and you don’t see any coolant entering the radiator, check your thermostat.

  • There is no universal rule for how much your engine overheats at 220 °F (104 °C), but most engines overheat above this temperature.

Measuring the Engine and Hose Temperatures

1. Start your vehicle to let the engine idle.

Please move your vehicle outside so exhaust fumes can escape. Turn on the engine and let it run throughout the entire test to heat up the coolant inside.

  • If you are concerned about your vehicle’s mobility, place blocks in front of each tire to keep it from moving.

2. Check the temperature of the engine with an infrared thermometer.

When you start your car, take an initial temperature reading as soon as possible. Locate the black hose that runs from the side of the radiator to the top or side of the main engine block. Point the infrared thermometer where this hose connects to the engine and pull outwards until it reaches maximum temperature reading. Wait until this number stabilizes before writing it down.

  • You can buy an infrared thermometer from a hardware store or online. This type of thermometer uses radiation to measure temperature, which is why it’s better than other types of thermometers because it doesn’t use mercury or other dangerous materials.
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3. Take the temperature of the upper radiator hose.

Take the temperature of the black radiator hose connected to the engine using a thermometer. Record the measurement you found.

  • If the temperature of the radiator hose is lower than the engine’s temperature, then there may be a problem with the thermostat.

4. Measure the temperatures again after 10-15 minutes.

Let your engine run for at least 10-15 minutes so that it can reach its optimal running temperature. Record your measurements from the same places you took them earlier and write down the results to see if they have changed.

  • If you see the engine temperature gauge on your dashboard begin to rise, be sure to turn off the engine as soon as possible to avoid overheating.

5. Check if the measurements are within 10° F (18° C) of each other.

If the engine and upper hose are both at a temperature near one another, then the thermostat is likely working properly. However, if the hose is significantly cooler than the engine, this indicates that there isn’t any coolant running through it and the thermostat may have stayed closed.

  • If the hose is at the same temperature as the engine, but your car is still overheating, there may be another problem with your vehicle.

Testing the Thermostat in Hot Water

1. Drain the coolant from the radiator.

Crawl underneath your vehicle and look for the drain plug. If it’s near the bottom corner or side of the radiator tank, place a bucket beneath it so that coolant doesn’t spill when you drain the radiator. Unscrew the plug, let coolant flow into bucket until your radiator is empty, and screw back on to tank when finished.

  • Vehicle coolant can be harmful to both children and pets if ingested. Keep it away from these vulnerable populations to avoid any potential harm.
  • If you’re going to drain the coolant, make sure your engine and vehicle are both cool before doing so. Otherwise, it could be very hot and cause burns.
  • If your radiator doesn’t have a drain plug, loosen the clamp around the lower radiator hose with a screwdriver before pulling it out of place. Drain the coolant from the lower hose into the bucket.

2. Disconnect the upper radiator hose from the engine.

After loosening the nut on the clamp, you can pull the hose off of the engine. Make sure to set this end of the hose aside so that you can access your engine’s thermostat housing later.

  • To prevent dirt or debris from getting inside the hose, place a shop cloth or rag at the end.

3. Remove the thermostat after disconnecting the thermostat housing from the engine.

The thermostat housing is a blocky metal piece attached to your engine block. The hose was attached to it with bolts, and you can remove the housing by loosening the bolts and setting them aside so you don’t lose them. Then, pull the housing off of the engine so that you can see the thermostat. It’s a small metal cylinder with a spring and two metal rings around it. Grab the end of the thermostat and pull it out

  • If it is difficult to grab the thermostat by hand, use a pair of needle-nose pliers or a screwdriver to pry it out.
  • If you have a thermostat with bolts that are different lengths, be sure to note where each one went inside the housing before reattaching it.

4. Submerge the thermostat in a pot of water so it doesn’t touch the bottom.

Fit the thermostat into the pot, submerging it completely. Fill the pot with water from a faucet and set it on a stove. Hold onto the top of the thermostat with tongs so that it’s submerged in water.

  • Don’t let the thermostat rest on the bottom of the pot since this could affect its temperature reading.
  • Hanging the thermostat on the side of the pot with a piece of string or metal clip allows you to more easily adjust the temperature.

5. Heat the water until it reaches about 195–220 °F (91–104 °C).

Turn the stove on to high heat and place a thermometer in the pot of water. Once the thermometer reads within 195-220 degrees Fahrenheit (91-104 degrees Celsius), turn off the heat.

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6. Take the thermostat out of the pot to see if it’s opened.

To remove the thermostat from the water and dry it off, use tongs to pull out the thermostat and set it on paper towels. As the thermostat heats up, the spring should compress and a gap will form between the center ring and outer ring. If there is a gap, then the thermastato is open; if not, then it needs to be replaced.

  • If your vehicle’s thermostat is not working, you can get a replacement from an auto supply store or online. Make sure the thermostat matches the model of your vehicle.

Finding the Thermostat

Finding the Thermostat
Finding the Thermostat

To find your car’s thermostat, start by locating the upper radiator hose and following it to where it enters the top of the engine. The thermostat housing will be located right where the radiator hose meets the engine housing, looking like half of a metal baseball.

Taking Out and Testing the Thermostat

Taking Out and Testing the Thermostat
Taking Out and Testing the Thermostat

If the thermostat is not responding to changes in temperature, perform the following procedure to get an accurate diagnosis.

  1. Disconnect the negative battery cable.
  2. Drain some of the coolant (a gallon should be enough) so the fluid won’t pour out when you perform the next step.
  3. Remove the upper radiator hose.
  4. Remove the thermostat housing.
  5. Remove the thermostat.
  6. Locate and write down the temperature stamped on the lip of the thermostat.
  7. Fill a pot with cold water and a thermometer and place on a stove burner.
  8. Place the thermostat into the cold water.
  9. Turn the burner on.
  10. If the temperature reaches the number you recorded in step #6, the thermostat will start to open. If it does not, or if it doesn’t open until reaching a different temperature (hotter usually), then there may be a problem with the thermostat and it should be replaced.

Why Should You Replace Your Thermostat?

If your thermostat becomes stuck, you may need to replace it.

Water Pump Failure Diagnosis

Water Pump Failure Diagnosis
Water Pump Failure Diagnosis

The thermostat is one of the most important parts of your car, and yet it is remarkably cheap to replace. Replacing it is usually much less expensive than repairing a warped engine.

Whenever the thermostat housing is removed, it’s a good idea to replace the gasket. A seal won’t be effective if the gasket has been used before.

Sometimes, instead of replacing a thermostat, car owners remove it and drive around without it. This is not a good idea because a cold engine does not function efficiently and consistently spending long periods warming up your car will take years off of the engine’s life.

Replacement of the thermostat

Replacement of the thermostat
Replacement of the thermostat

Thermostats can cost anywhere from $150 to $370 in parts and labor, depending on the car. In some cases, replacing the thermostat requires more difficulty accessing than others – requiring additional time and/or labour. When a new thermostat is installed, it normally comes with a new gasket as well. The cooling system must be properly bled when the thermostat is replaced in order to remove any air pockets and ensure proper engine function.

To ensure proper cooling, it’s important to check the fans come on when the engine is warm and the car can provide a good idle heat. If there is no idle heat, this could mean there are air pockets in the cooling system.

It is not necessary to replace the thermostat on a regular basis, but it may be a good idea to do so when the engine is rebuilt or replaced.

Some vehicle manufacturers recommend updating the engine computer software when there is a Check Engine code related to the thermostat, after it has been replaced. You can get a subscription fee-based access to a factory repair manual in this article.

How a car’s thermostat works

How a car's thermostat works
How a car’s thermostat works

Thermostat is closed

When the engine is cold, the main thermostat closes, stopping coolant from flowing through the radiator. The smaller thermostat bypass valve is open, allowing coolant to circulate through the bypass tube within the engine and through vehicle heating system. This allows the engine to warm up faster.

Thermostat is open

As the engine warms up, the thermostat valve gradually opens up, allowing coolant to flow through the radiator.

The coolant flows from the radiator through the system and into the engine. The water pump pulls this cooled fluid back into the engine.

If the engine’s temperature falls within a certain range, the thermostat will close to maintain the temperature.

Many modern cars use an electronically controlled valve called a “coolant control valve” in place of a conventional thermostat. The coolant control valve works the same way as a traditional thermostat, only it is operated by the engine computer and is more precise.

F.A.Q about “how to tell if thermostat is stuck open”

What will happen if thermostat is stuck closed?

If the thermostat becomes stuck in the closed position, it will prevent coolant from circulating around the engine, causing it to overheat.

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How do you unstick a closed thermostat?

If you have a cold car, it can be difficult to unstick the thermostat. To start, turn your engine off and let the car idle for 10-20 minutes. This will help your vehicle reach its optimal operating temperature and make it easier to remove the thermostat.

Can you fix a closed thermostat?

If the hoses do not show any blockages or coolant hindrance, it may be time to replace the thermostat.

Does a thermostat fail open or closed?

Thermostats are devices that regulate the temperature in a car. If one of these thermostats fails, it can cause problems with the car’s cooling system or even its engine.

How can I test my thermostat?

How to Test an Old Thermostat
  1. Step 1: Turn off HVAC power. This can be done by switching the furnace to the off position if your system has an on/off switch. …
  2. Step 2: Remove thermostat cover. …
  3. Step 3: Remove wires from terminals. …
  4. Step 4: Turn the HVAC power back on. …
  5. Step 5: Test other wires.

Will heater work if thermostat stuck closed?

If the thermostat is not opening properly, the engine may overheat and fail. If this happens, it will run on the warm side and not past halfway; however, it will still be able to heat up the interior.

Can a car thermostat unstuck itself?

If something goes wrong with your car’s engine, it’s possible the thermostat may become stuck in the open position.

How do I know if my thermostat is working?

Make sure the screen on the thermostat is lit and that the batteries are fresh. If either of these conditions are not met, a thermostat failure may be the cause.

Can a thermostat be partially stuck?

If the thermostat’s piston rods get stuck halfway or if the wax gets degraded, it can result in a condition where the valve isn’t able to open all the way. This can cause mild symptoms that occur when both conditions are present, but they’re less severe and immediate.

What causes a thermostat to stick closed?

If the car’s cooling system isn’t working properly, there are a few likely causes. One of the most common is a blocked radiator, which can be caused by low water levels or a failed cooling fan. There are also more involved reasons that include a failed/ failing water pump, an internal obstruction in the radiator, or even the engine itself.

How do I know if my thermostat or water pump is bad?

If you experience any of the following five signs, it’s time to bring your car in for inspection and possible water pump repair:

1. Your car is making a strange noise or vibration when driving.

2. You see rusty or algae-covered areas on your car body.

3. The engine starts bogging down while trying to accelerate or go uphill, even after adding fuel and driving normally for a while beforehand.

4. Water randomly starts coming out from under the hood, even when there’s no rain or snow outside currently . . .

Can a stuck open thermostat cause rough idle?

A stuck open thermostat can cause your idle to go haywire. Low coolant temperature can cause the Idle air bypass valve to stay open, which will raise the idle. This could change while you’re driving, and under a steady load or at idle for a long enough time, the engine might get up to temperature.

What happens if engine runs too cold?

If you run your engine cold all the time, it is likely that you will experience increased fuel consumption across the board. Additionally, carbon buildup will be more prevalent at various parts of the engine, causing more damage.

How can I test my thermostat without removing it?

How do I fix an unresponsive thermostat?

If the thermostat is still unresponsive, make sure all breakers are off and check for any obstructions that may be preventing heat from reaching the unit. If it looks dirty inside, use a can of air or a soft brush to clean away accumulated dust and grime. Look for issues like loose wiring or screws that may be preventing the thermostat from working properly. Once you’ve identified any problems, fix them accordingly.


Hopefully, this article was able to help you determine if your thermostat is stuck open and provide you with a few different ways to determine the problem. If you have any questions, don’t hesitate to contact us. Thanks for reading!

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