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How to Determine If a Car's Engine Thermostat Is Stuck

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Ms. Millar has had many books published online and in print. She learned how to bind her own books to save money on publishing costs.

The thermostat that fits in Toyota A, E, S and C engines.

The thermostat that fits in Toyota A, E, S and C engines.

What Does the Engine Thermostat Do?

If a car's engine is too hot, it can actually warp and become damaged. But, when it's cold, it doesn't run efficiently. So, it is important to keep the engine at the right temperature, which is where the thermostat comes in. An engine's thermostat regulates the temperature of the engine by controlling coolant flow.

The coolant does exactly what you imagine—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 the car's operating temperature (usually either 180º F (82º C) or 212° F (100°C). While the thermostat controls coolant flow, the radiator's job is to cool the fluid. The coolant does not flow through the radiator unless it is hot and needs to be cooled.

  • When an engine is cold, the radiator fluid is cold, so the thermostat is in the closed position. In this position, coolant will only flow inside the engine. It will not be able to leave the engine to cycle through the radiator to be cooled.
  • When the engine starts up, the coolant warms. When the coolant reaches a specific temperature, the thermostat opens. Each thermostat is gauged to open at a specific temperature. Once it opens, the coolant can circulate through to the radiator to be cooled.
A thermostat in open position and a thermostat in open position

A thermostat in open position and a thermostat in open position

Possible Signs of a Bad Thermostat

  • The car overheats easily, or the radiator fluid steams or bubbles. A stuck thermostat is only one possible cause of a car overheating (but it can be the easiest cause to fix).
  • The heater doesn't provide warm air even after the engine has been running a while (though there are other possible causes including a bad heater core).
  • You see a "check engine" light.
  • The temperature gauge stays on "cold" all the time.

Will Replacing the Thermostat Fix My Problems?

You might hope so, because the thermostat and housing together cost less than $20 and are not difficult to replace. Keep reading to find out how to tell whether your thermostat is faulty.

How to Tell if Your Thermostat Has Stopped Working

When the thermostat gets stuck and stops working completely, there are several indicators you can check to determine the culprit. See the table below.

3 Ways You Can Check Your Thermostat's Operation

Symptom TestExplanation

Cold engine test: Open the hood and remove the radiator cap. Start the engine. Observe coolant action by looking down into the radiator. The fluid should not be moving.

When an engine is cold, the thermostat should be closed, thus bypassing the radiator.

Warm (not hot!) engine test: Open the hood and remove the radiator cap. Start engine. Look down into the radiator and observe coolant action. The fluid should be swirling vigorously.

When an engine is warm, the thermostat is in the open position. This allows the coolant to flow through the radiator causing vigorous movement.

Cold engine test: Start the engine. Turn on the heater. Time how long it takes the engine to warm up enough for the heater to actually produce heat in the passenger compartment.

If it takes more than five minutes for the heater to produce heat, it is a sign that the thermostat is stuck open, allowing the coolant to detour through the radiator all the time. This causes it to take a good deal longer for the engine to warm up.

The Other "Radiator": Your Car's Heater Core

You know where your car's radiator is, but there's another component, under the dashboard that looks and acts like a small radiator: it is called the heater core. When the car is warmed up and the heater is turned on, a fan blows air over the heater core. The heat produced by the hot radiator fluid runs through the heater core and is dispersed into the cabin of the car, heating the car's interior.

The thermostat on a Chevrolet 454

The thermostat on a Chevrolet 454

Locating the Thermostat

You can find your car's thermostat by finding the upper radiator hose and following it to where it enters the top of the engine. The thermostat housing is right where the radiator hose meets the engine housing. It looks like half of a metal baseball.

Removing and Testing the Thermostat

If you are still unsure whether the thermostat is stuck, 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. Watch the thermometer. When the temperature rises to 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 the thermostat is bad and should be replaced. If it opens at the designated temperature, then the thermostat is just fine.

Why Replace Your Thermostat?

When the thermostat becomes stuck, it is reasonable to simply replace it.

The thermostat is such a vital component to the well-being of your vehicle, yet it is so cheap. Replacing it is far less costly than repairing a warped engine. A warped engine must be removed and either resurfaced, if the damage isn't terribly bad, or completely replaced.

And whenever the thermostat housing is removed, even just to check the thermostat, you should at least replace the housing gasket. A housing gasket needs to be able to seal fully, and it won't if it's been used.

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.

Be safe and replace!

Thanks for stopping in!

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: My car is not running hot, but I have no heat going inside my car. Is this my thermostat not working?

Answer: It's your heater core or the fan that blows across the heater core. Replace the heater core and/or fan and you will have heat.

Question: I let my car warm up, and when I get going, I'll turn my heater on and then my temperature gauge goes back down to cool. Then, I'll turn the heat off, and the gauge goes back up. What could be happening?

Answer: This anomaly is normal. In some vehicles, it is more obvious than in others. Some drivers use this anomaly to aid them if the car starts to overheat. To understand the cause a little cooling system background is in need. The heater in a vehicle uses the radiators cooling system. Yes, really. The radiator fluid is routed through the heater core in your vehicle and back to your engine. Check back for a hub entitled Vehicle Engine Cooling System. It will go into depth about what you are experiencing.

Question: My temperature gauge is between midway and cold and never seems to get hot but I have heat in the cabin. Is my car's thermostat going bad?

Answer: Between midway and cold sounds like a great reading to me. Is it normally hotter? If so, yes, check your thermostat. As long as the engine is running cool that's fine. You don't want it running cold, or hot. Anywhere in between, especially towards the cold side is good.

Question: Why is the overflow for my radiator in my Blazer boiling?

Answer: What is your thermostat reading when this happens? Water boils at 212 F. The thermostat opens at about 180-200 F. If the water is boiling in the overflow tank that means it is at or above boiling temperature of 212F. Check your thermostat for proper operation and top off your radiator fluid.

Question: I have heat in my car but my car is bubbling by the radiator and steam comes out of my car it's not overheating could it be the thermostat?

Answer: Absolutely. It sounds like the thermostat is stuck in the closed or semi-closed position.

Question: I have a 2015 Toyota Corolla, the heater is only hot on one side the other side remains cold?

Answer: If I understand correctly the heater only works on one side of the vehicle and not the other. That sounds very odd. Check that the coolant level is full and there are no air bubbles in the cooling system. If the coolant isn't full it can cause poor heating inside the vehicle.

Question: Can a thermostat that opens and close when put in hot water still become stuck at close when installed in the car? I am experiencing this situation.

Answer: It is unlikely a problem with the thermostat if it has proven to open and close fine in boiling water. What was the temperature of the water when it did open would be my question? Have you checked for foreign debris? If there is an obstruction blocking the thermostat from moving such as rust buildup at the site of installation. Also, there may be foreign material in the radiator fluid which becomes caught in the thermostat when the engine is running.

Question: My temperature gauge only moves up to the 1/4 mark on after driving 20 miles. Shouldn't it be half way by then?

Answer: No. The halfway mark on the temperature gauge is not the "go to" position for your car. Some vehicles temperature is regularly at a quarter, some at just below the half mark, or somewhere in between. As long as the gauge does not go above the half mark your engine is fine.

Question: My car only overheats when in traffic or at idle. My radiator fans do not come on unless I turn a/c on. What could the problem be?

Answer: If you do not turn on the a/c will the engine overheat where you have to pull over? Sometimes the engine will get very warm but it's okay. When the a/c is turned on it will push the engine too hot, thus engaging the fans. A/C is notorious for pushing engines too hot. Where yours is overheating enough to kick in the fans only when the a/c is started. Also, check that your thermostat is in working order.

Question: My 2010 Buick Enclave got hot too fast and my dashboard is saying "reduced power to engine". The coolant is in the car but not working obviously. What should I do?

Answer: The first thing I would check is that the coolant level is indeed full in the radiator. With the engine cold, open the radiator cap. Turn the engine on. Watch the radiator fluid inside the radiator where you removed the radiator cap. The fluid should wiggle some at first, after a few minutes, when the engine warms up, the radiator fluid should being jostling quite a bit due to the water pump working. If the fluid never jostles around the water pump is not working. Next, I would check the thermostat. If the thermostat doesn't open the engine can overheat.

Question: Why does the temperature gauge climb high while I still have heat?

Answer: The gauge continues to climb because the engine is overheating. The heater is not related to the reading of this gauge.

Question: My temperature gauge, in the car, reads normal, and the motor doesn't over heat. But I don't have heat in the car. Why is this?

Answer: When you turn the heater on high can you feel the air blowing into the cab of the vehicle? If yes, you can feel air blowing, but it is cold air, then look to your heater core. The heater core is where the heat comes from. If you do not feel air blowing then your heater fan, or control panel, needs to be replaced. Check the heater core first.

Question: I have a 2014 Dodge Caravan, the heat is working and the temp gauge doesn't go past half point. However, my check engine light came on with the code p0128 for the thermostat. I don't understand why it has not overheated and I still have really good heat inside. What could this mean?

Answer: The thermostat may be working inappropriately. Have you pulled the thermostat and boiled it in a pot to check when and if it is opening? A vehicle can run just fine if the thermostat doesn't open. It will run on the warm side, not past halfway, and heat the interior just fine. But, when the weather gets warmer outside that thermostat stuck closed will not be able to allow the reservoir water flow and the vehicle will begin to have overheating issues. My first step would be to pull the thermostat, boil it and make sure it is opening at the proper temperature (usually marked on the lip of the thermostat.

Question: Would a bad thermostate cause some heat only on one side of vehicle other side cold?

Answer: I wouldn't suspect the thermostat. I'd look into a blockage in your cooling lines. Coolant isn't reaching one side of the engine. Check to see that the hoses are in place, too.

Question: How do I check to see if the thermostat is stuck closed, and what are the possible effects of this?

Answer: One simple way to check the thermostat is to remove the housing, which is usually located on the top, front and near the center of the engine. Then, remove the thermostat for inspection. Is it rusty? There's a small stamp on the thermostat reflecting the temperature it should open at. Next, put it in a pot of water and bring the water to a boil. Does the thermostat open when the water reaches that temp? Effects: slow warm-up and overheating.

Question: My heat isn’t working and my car started to overheat but the last time that happened it was because my radiator had a crack in it so I replaced it. What could be wrong with my car now? I have an 06 Acura tl

Answer: You may want to check the heater core. It is a radiator inside the car that produces the heat when you turn the heater on. Have you noticed a smell of radiator fluid in your car? Is the windshield getting foggy for no apparent reason? If the heater core checks out then check the water pump. With the radiator cap off, turn the car on. Watch for the water flow inside the radiator. Is it swirling around vigorously as the car warms up? Or is it not moving much at all? As your vehicle warms up the thermostat opens up and the water should start churning around as the water flows through the water pump cooling the engine.

Question: How do I fix a 1999 Honda Accord Lx if it is overheating into the overflow jug, and the thermostat quit working inside the car? What do you think it is?

Answer: Check the heater core. It may be blocked or leaking.

Question: I have a 2000 Honda Civic. Can a thermostat that is stuck closed cause an old radiator to crack?

Answer: Not necessarily. It would crack from overheating or freezing. The thermostat could be the culprit, but low coolant would be more likely. Coolant not only cools the engine it also prevents the fluid from freezing. Do you have coolant in your system or just plain water? Water will freeze, expand, and crack engine parts. That's what freeze plugs are all about. They are supposed to pop out before your engine cracks if it becomes too cold.

Question: My car's temperature gauge on dash is not reading, but my heater is still blowing hot air. Do you think it might be the coolant temperature sensor that went bad?

Answer: That wouldn't be my first choice. Check the wires first. Start at the gauge and make sure the wires haven't been pinched, detached, loose or broken.

Question: The lower hose came out in my car while driving and then got it replaced, but now the temperate gauge does not move at all and does not have hot air either. Is this because of the thermostat?

Answer: It sounds like your thermostat was adversely affected when the hose fell out. It probably ran the fluid out, causing the thermostat to run dry which in turn froze it in one position.

Question: When my car is idling the temperature gauge rises as normal to midway. When I start driving, the temperature gauge drops quickly back to zero. Is this a thermostat problem? The heat inside works.

Answer: It sounds more like a defective gauge to me. When the car starts to move, it sounds like the signal is lost to the temp gauge. I am also thinking maybe when you start to drive the thermostat opens and dumps all the cool radiator fluid into the engine causing it to cool down, but I don't believe it could cool it that much that fast. I'd check your connections first.

Question: Just replaced temperature sensor on my Audi. After a month of the car running cold, mostly on the freeway, but when going slow as well, it stays warm. It takes a too long to warm up. Is the temperature sensor stuck open?

Answer: The temperature sensor detects the temperature of the engine. There is no "stuck" about it. It sounds like you are having a problem with the thermostat. Is that what you were referring to when you said temperature sensor?

Question: If I have a 98 Dodge Ram 5.9 my gauge stays between cold and halfway after warm up but the heater only blows cold air could this be the thermostat stuck open?

Answer: If the thermostat was stuck open the heater would eventually warm up. In between cold and halfway sounds like a fine engine temperature. This is more likely a problem with the heater core.

Question: I have a 2015 ram 1500 3.6 v6 right now it’s about 1 Celsius outside. It takes my truck about 30 minutes of highway driving to warm up. Could it be that the my thermostat is stuck open?

Answer: Absolutely.

Question: My truck is not leaking fluids, steam is not coming from the radiator but the cap is stuck on and the temperature gauge is at 220, is this the water pump or the thermostat?

Answer: If I understand your question, the radiator cap is stuck and your gauge reflects 220 degrees? The radiator cap is "stuck" because there is a huge amount of pressure behind it. 220 degrees is overheating. Overheating causes pressure. The pressure presses on the radiator cap, thus making it very difficult, if impossible to remove because you shouldn't remove it. Allow the temperature to reduce and the cap should come off by pressing down and twisting clockwise. Do not attempt to remove the cap when the temperature is high!

Question: I just put together a 462 big block Chevy for drag racing. The engine is running hot, the thermostat is working, but when I removed it, there is no coolant in the housing, shouldn't there be some fluid in there?

Answer: Yes, there should be coolant unless its running low, leaking out, or simply not enough to see it in the bowl. If the thermostat is atop the engine when you open it up the fluid may be just below it.

Question: My car's thermostat isn’t moving at all, what does that mean?

Answer: If your thermostat is not moving, even when the ignition key is turned to the ON position, you have a wiring problem. Somewhere the wire sending the signal to the dashboard is broken, or the fuse is blown.

Question: My heat gauge is slightly past the middle and stays like that on short trips. When I drove 50 miles, the heat gauge started going towards hot. Any idea why?

Answer: My question for you is, "How far towards hot?" Did you have to stop for fear of over heating? If it just went a little towards the hot side then it was merely due to continued work of the engine causing it to become warmer than mere local driving. If it went a lot towards hot, but not over heating, check your antifreeze fluid level, it may be low.

Question: Recently my top radiator hose came off while I was driving. I'm guessing it was loose. The antifreeze had spilled out when the hose came off, so I walked to the auto parts store to buy more. When I put the hose back on and added the antifreeze, my temp gauge is high, but it is not overheating. Why would the temp gauge go down when I step on the gas pedal?

Answer: There may be something stuck in the coolant lines or your thermostat is bad and when you hit the gas it's forcing it to open which allows the coolant to flow. When you release the pedal it stops flowing and heats up again.

Question: My heat only works when the vehicle is going down the highway, it cools down when I slow or stop. Could I just replace the thermostat and it will be fine or is it more than that?

Answer: Sounds like the heater core is not working properly. Check to see if the radiator is low on fluid, if not then I would check your heater core.

Question: When I am On the highway my car heater works fine but when I slow down it blows out cold air and the temperature gauge rises. What could be wrong with my car?

Answer: The wind created when driving on the highway is what is keeping your engine from overheating. When you slow down the heat becomes cold most likely from low radiator fluid.

Question: There is no coolant leaking and the oil color is normal, but my temp gauge stays on cold. Would these symptoms indicate a stuck thermostat?

Answer: Yes, it definitely would be a stuck thermostat.

Question: My car started heating up. So I changed the thermostat. It kept on heating up. So I used the car without the thermostat all summer. It was good. What should I do about my overheating car?

Answer: I have an old truck I pulled the thermostat from and still use it without it. It will take your engine longer to heat up without it. Try a new thermostat with fresh radiator fluid, or leave it out.

Question: My thermometer takes about two to three minutes to rise up to par. When the car is stopped like at a red light or something, the thermostat starts rising up to over heating. But when I start driving it goes back down to the middle. What's going on?

Answer: Check the fluid level in the radiator. Add radiator fluid if necessary. If there is not enough fluid running through the system when you are stopped the temperature of the fluid goes up. When you drive the wind blows across the radiator cooling the fluid down. When there is not enough fluid in the system it cannot keep the temperature down without assistance from the wind passing over the radiator.

Question: My car is not warming up quickly when I start it. I have to wait for 7 to 10 minutes for it to warm up otherwise I cannot drive it. Is this a thermostat not working?

Answer: Yes, it sounds like the thermostat is not closing properly. An engine warms up fairly quickly because the thermostat restricts the flow of antifreeze to only the engine. Once the engine is warm the thermostat opens allowing all the antifreeze to flow. It sounds like your thermostat is staying open, thus allowing all the antifreeze to flow, all the time, making the warm up process slow.

Question: My car temperature goes up to overheating within five minutes. With the heat on the air begins to warm slowly like the thermostat is stuck open. Why would my car overheat so fast if the thermostat was open?

Answer: It may be because you are low on radiator fluid.

Question: My thermostat is stuck closed (i.e. it's stuck at 50) and my AC is no longer working. Could my A/C failure be only a thermostat problem?

Answer: I'm not sure what is meant by "it's stuck at 50". Is it 50 degrees it's stuck at? A closed thermostat will cause an engine to overheat. The A/C unit is sometimes the first to shut down on such an occasion as overheating. Try replacing the thermostat first, it's the cheapest and fastest fix to see if there us a further problem.

Question: Why is my car's engine leaking coolant after I replaced the thermostat?

Answer: Check the hose clamps. Something is loose. If you are positive all the hoses are secure, check the hoses themselves. There may be a crack in a hose line.

Question: It takes my 2015 ram 1500 30 minutes to warm up at -5 Celsius out side. Stuck open thermostat?

Answer: Yep. You may want to check the rating on your thermostat as well. There are various temperature settings better for Summer and others are better for Winter.

Question: My fan runs as soon as I turn my car on and my temperature gauge is staying on cool. Is this a sign of a bad thermostat?

Answer: Not necessarily. I am assuming your vehicle uses an auxiliary fan and not a clutch fan. Before blaming the thermostat check and make sure the fan itself doesn't have any problems.

Question: I have no heat in our van even when the temperature and engine are warm. Sometimes the engine almost runs hot, then all of a sudden, the temperature goes to normal and we have heat. Could this be a thermostat issue?

Answer: That sounds very much like the thermostat is the culprit.

Question: My car is running hot, but I have no heat going inside my car, is this my thermostat not working?

Answer: This would be a heater core issue. The heater in your car uses a device called a heater core. It is very similar to a radiator, but it is located under your dashboard. Heated radiator fluid flows from the engine into the heater core and back out to the engine. When the heater is activated in your car a fan blows across the heater core causing the heat from the radiator fluid to flow into the cab. Either the heater core is bad or the fan that blows across the heater core is not working.

Question: I'm from the UK. I have an Astra H Mark 5 09. I have been getting a fault code, which suggests the thermostat is faulty, but I don't have any symptoms of a faulty thermostat. Could it be the heating element that is gone bad?. The problem I have is when I cold start the car, it starts the first time. When I turn it off or go for a short journey, it won't startup. I must turn the key twice. It always starts on the second attempt. Any suggestions?

Answer: Hello to you in the UK. I am not familiar with the Astra H. Just knowing the starting system in vehicles I would look at the starter first for sings of going out then like you said, check the heating elements, especially the one on the exhaust. Heating elements build up filth fast when on the exhaust.

Question: How long does it take to warm up the engine of a Corolla hatchback after the engine is turned on?

Answer: That depends on several variables IE: What is the weather? Cold, Warm, Hot? Is the car being driven right away or parked while it warms up? Is the radiator full of fluid? If everything is on a medium basis, warm weather, parked while warming, full of fluid, I would say within 3 - 5 minutes it will up to warm.

Question: How do I change my thermostat housing on a 2004 Toyota Tacoma V6 ??

Answer: This article reflects on the 2007 Mustang, but the process is similar across all vehicles. It will walk you through the thermostat housing change.

Question: The fan in my car stays on. I've changed the temperature switch. What can be problem?

Answer: If you are referring to the radiator fan there could be a few different reasons for this. Some vehicles are equipped with a fan that remains on even after the engine is turned off to continue cooling the engine. It eventually turns itself off. Another reason may be a wire is touching ground, thus continually sending the signal to the fan to keep running. If it's an older vehicle the fan clutch may have gone out and needs to be replaced.

Question: I have Dodge Journey 2009. My heater is not working. It is giving cool air only and there is also a check engine light on my dashboard. Could the problem heater in my Dodge Journey be a stuck thermostat?

Answer: Make sure the radiator fluid is full with no bubbles. Squeeze the upper radiator hose to press out any air bubbles that may be trapped in the upper hose. The check engine light comes on when the smog system has a problem. Check the EGR valve.

Question: If my car overheats it causes my fan to stop working. This happened as I was driving. What you think the cause of my car's overheating can be and what is the best way to fix it?

Answer: If I understand your question, the radiator fan turns off when your car overheats. Check to be sure if it isn't the opposite way. The car may be overheating BECAUSE the fan turned off. If the fan turns off an engine will overheat quickly. If the overheating is causing the fan to turn off the check that there is no wiring that is becoming hot and causing a ground which in turn could turn the fan off.

© 2012 Joanna