Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.
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 Does a Car Thermostat Do?
The Thermostat at Work
Bad Car Thermostat Symptoms
Common Reasons for Abnormal Engine Temperature
How Do I Know If My Car Needs a Thermostat?
Checking Coolant Flow
Checking Coolant Temperature
How to Test a Car Thermostat
What Does a Car Thermostat Do?
As any other water-cooled engine on a passenger vehicle, the engine in your car operates at a temperature range of approximately 195–220 °F (91–104 ºC). To help it operate within this range, your car 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.
Symptoms of a Bad Car Thermostat
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 Causes of Abnormal Engine Temperatures
- 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
Read More from AxleAddict
How Do I Know If My Car Needs a Thermostat?
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.
How to Check Coolant Flow
- Wait for the radiator and engine to cool.
- Engage the parking brake and block the wheels.
- Remove the radiator cap, fire up the engine and let it idle.
- 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.
- If the coolant is not flowing, go to step 7.
- 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.
- 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.
- 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.
- 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.
How to Check Coolant Temperature
A simple way to check for coolant temperature, and hence, thermostat operation, is to use your own hands.
- Engage the parking brake and set your transmission to Neutral (manual) or Park (automatic).
- Start the engine and let it idle.
- Touch the upper radiator hose and lower radiator hose, so that you can feel the hoses' temperature. Be careful with engine moving parts when doing this.
- Wait about 10 minutes and repeat step 3. You should notice the hoses' have risen in temperature. This means the thermostat is opening. If the hoses' temperature have remained about the same, the thermostat is stuck closed.
If you want, you can do a more accurate test. For this test, you can use a needle type, kitchen thermometer or an infrared thermometer type.
- Make sure the radiator and engine are cool, engage the parking brake and block the wheels.
- Start the engine and let it idle.
- 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.
- After five minutes, read the temperature again on these same two test points and compare these readings to the previous ones.
- Wait for five more minutes and take another reading.
- 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.
Interpreting your results:
- 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 and 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 stuck closed thermostat.
- 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.
How to Test a Car Thermostat
Checking your car thermostat is the best way to know if the unit has actually failed. The testing procedure requires a kitchen thermostat and a pair of needle nose pliers. First, you need to remove the thermostat from your car.
The thermostat housing is 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:
- Visually inspect the thermostat. It should be in the closed position. If it is opened, replace it.
- Place a kitchen pot on a stove. Pour enough water into the pot to cover the thermostat.
- Submerge the thermostat in the kitchen pot, but don't let it touch the bottom of the pot. Use a pair of needle nose pliers for this.
- Start heating the water and place the kitchen thermostat in the pot, but don't allow the thermostat to touch the pot. You just want to monitor the water's temperature.
- Watch closely and notice at what temperature the thermostat begins to open.
- Make a note of the temperature at which your thermostat began to open.
- Wait for the thermostat to open completely and make a note of the temperature. Then, remove the thermostat from the pot and check that the thermostat gradually closes completely.
- Compare your notes to the thermostat operating temperature specifications in your vehicle repair manual. If your thermostat deviates from the specifications, or the thermostat mechanical action deviates from the one described above, replace it.
Most vehicle engines operate at a temperature range of approximately 195–220 ºF (91–104 ºC). Check your vehicle repair manual for the thermostat operating range, and make sure you have the correct one for your application. Or replace it, if necessary.
The next video gives you an idea about replacing a thermostat in a car.
How to Replace a Bad Thermostat
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 tell 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.
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: I have a 2013 Toyota Yaris model and started overheating after one hour drive on 120 speed. What could be the problem?
Answer: There could be several reasons for an engine to overheat. A faulty thermostat is only one of them. This other post can help you diagnose and, possibly, fix the problem.
Question: My truck won't start after it overheated what should I check?
Answer: If it still cranks, check for spark and fuel delivery.
If it doesn't crank, remove the spark plugs and see if it cranks. If it doesn't but the starter motor is engaging probably there's damage to the head gasket or engine.
Question: Why does my low coolant light stay on?
Answer: If the coolant level is OK, and the engine is not overheating, check the coolant level sensor.
Question: My car is overheating, but the problem is that it sometimes overheats fast and sometimes after 25 minutes of driving. What could be the cause of my car's overheating?
Answer: There could be a problem with the water pump. The bearing or impeller might be worn and not working constantly. If the pump is run by a drive belt, check the belt's tension as well.
Question: I'm not getting any heat inside my 2003 Jeep Liberty. What could cause an early 2000s Jeep to not heat properly?
Answer: Make sure there’s enough coolant in the system. Also, your heater core could be plugged and not allowing coolant to circulate, or an actuator might be bad.
Question: My 2003 Hyundai Sonata runs great, but the temperature gauge is always in the middle of cold and hot. Is it overheating?
Answer: If the gauge is not in the red zone, it might be working fine. Otherwise, check that the fan for the condenser is working fine when turning the AC. Other potential problems could be a failing AC compressor, fault in the cooling system, or a bad coolant temperature sensor.
Question: I just replaced my thermostat and now my car is over heating, and it wasn't doing that before. Do you know what could be causing my car to overheat?
Answer: There could be air in the system. Check your repair manual. If you don't have the manual, this post may help:
Question: after how many miles should a thermostat be replaced?
Answer: Usually, there's not a time frame to replace the thermostat, but keep an eye on it after five years.
Question: Where do I put the coolant in my car?
Answer: Remove the radiator cap and pour the coolant through the opening. Follow the instructions in your car owner's manual or repair manual. You'll need to purge the system.
Question: I have a 2005 Chrysler 300 Touring. When I was cutting my air on, it started to overheat, but today I purchased this bars seal, and it leaked from the head gasket. Now my car is running hot whether the air is on or not. What do you suggest the problem is? I purchased the 32 dollar bottle. What do you think the problem is?
Answer: If the head gasket is leaking coolant, it is not circulating properly in the system. Probably the bars seal won't cut it. You'll need to replace the head gasket.
Question: I have a 96 Mustang. The temperature gauge will get just under the red mark and then drop down to the normal range. What could be wrong with my temperature gauge?
Answer: Common possibilities: the coolant temperature sensor or water pump is going bad, but this can also be caused by a clogging radiator. A malfunctioning water pump can create hot spots and trigger the gauge to go up.
Question: What happens if I remove the thermostat from my car?
Answer: Without a thermostat, the engine will never reach the correct operating temperature. Besides an engine running below efficiency, engine components wear will accelerate. So, it's not a good idea to run an engine without a thermostat.
Question: Can I drive a short distance with a faulty thermostat?
Answer: Just keep an eye on the temperature gauge. When you see the needle reaching the red line, pull to the side and wait for the engine to cool, if necessary. What you want to avoid is engine overheating.
Question: The engine is not overheating and everything else is working fine. However, the engine fan comes on as soon as I switch the car on. Will this cause problems as the winter sets in?
Answer: Probably the engine coolant temperature switch, relay or wire in the circuit is faulty. Try unplugging the switch and start the engine. If the cooling fan doesn't come on, is probably the switch. If it keeps running, a relay may be stuck. You may want to check the diagram for your particular model and see if there's a relay in the circuit. And check the wiring. Although rare, a control unit may give you trouble as well. This other post may help:
Question: How long after replacing the thermostat does it take to get heat up?
Answer: The engine usually reaches operating temperature in about 15 to 20 minutes. That's when the cooling fans kick in.
Question: Why am I getting water in my spark plugs on my 2001 Ford Expedition?
Answer: Internal leak: A leaking head or intake gasket could be the problem. Other possibilities include a cracked head or block.
External leak: On some Ford Explorer models the cowl can actually leak rainwater into one or more plugs. Water seeps through the well and can cause a misfire. Haven’t heard of this same issue on Expeditions. But check the boot and well for signs.
Question: I have a Toyota Corolla 2005 I'm having heating and air problems. The heat blows cold air in the winter, and now that it's getting hot the cold air wants to blow hot now. All of my vents and everything works completely fine, and it's filled with coolant. Could opening the thermostat cause this issue?
Answer: Check the temperature gauge - unusual readings may indicate a bad thermostat. You may want to check the refrigerant level and condition, and flow valve.
Question: What is the problem when very hot air is blowing on the hot or cool dial?
Answer: Check the refrigerant level. This issue also may be caused by a faulty condenser or some other issue in the system. This other post may help:
Question: Does a thermostat in a car have a switch?
Answer: The thermostat itself has a wax capsule that works as a sensor/switch, but there’s probably a cooling fan thermostat temperature switch sensor.
Question: I have 2006 Honda Civic. Temperature gauge shows normal reading when fan off, but when I turn the heat on, the temperature gauge drops to minimum. AC works fine. What could be the reason?
Answer: You probably need to replace the thermostat.
Question: Can just the housing on a car thermostat be replaced?
Answer: Yes, you can replace the thermostat housing. Go to your local auto parts store or order online. You’ll need the vehicle make, model and engine size to get the correct part for your application.
Question: I have a 2005 Chevy Express 1500. I changed the coolant sensor and the thermostat but my temperature gauge is still on cold. What can be the problem at this point?
Answer: Check the wiring and connection between the gauge and the sender or switch. You may need the wiring diagram for your model and a test light or digital multimeter. You can find in your vehicle repair manual. If you don't have the manual, stop by your local auto parts store. This other post may help:
Question: I have a 2008 PT Cruiser. I checked the radiator cap on it. It was bad so I replaced it. I thought that was the problem. My temperature started to rise, so I cut the car off, let it cool for a while, then put water back in the radiator. Then I noticed coolant was leaking from the top of the radiator but the radiator seems fine. Could a faulty thermostat cause it to do that?
Answer: Check the radiator neck; another possibility is the radiator tank.
Question: How do I know what thermostat should I use for a 2008 Chrysler PT Cruiser? It seems that there are some that can resist more temperature.
Answer: The thermostat should be calibrated for your specific model. Just ask the guys at the auto parts, they should give you the right one for your application. If necessary, use your car's VIN number to avoid any confusion.
Question: I have a Toyota 80 series Cruiser. I hooked my float up to go to an event. Drove 30kms and it overheated. I drove home unhooked and drove 130kms to the event with no float no problems. What could the issue be?
Answer: Check the fan hub, probably the oil is leaking or there's something wrong with the assembly. This is usually the problem.
Question: I have a 2000 BMW E46 and every 2 months the expansion tank cracks. Do I have to replace the thermostat?
Answer: This type of expansion tank is under pressure. Make sure the tank's cap is working properly and venting. Still, there might be an overheating problem or/and pressure issue. There could be several reasons for this. Make sure you are not overfilling the system. Consult your vehicle repair manual. A bad thermostat, cooling fan, water pump can cause the problem. Also, too much pressure is getting to the tank from; sometimes, a faulty (cracked) engine block, head gasket or cylinder head. This other post may help:
Question: I have "Top-up Coolant" warning light coming on an off sporadically in my car. The garage has charged me for a turbo valve but the problem is persistent. I am convinced it is a thermostat issue but no one will believe me as my vehicle is not showing any other faults in a diagnostic test. Can you please help?
Answer: Check the coolant level sensor. There could be something wrong in the circuit or the sensor itself. You probably can do it if you have the repair manual for your model.
Question: How critical is replacing the thermostat casing?
Answer: If the thermostat housing is worn, damaged (cracked) or has beginning to leak, it's better to replace it. Coolant uses the housing as a flow-connecting point into the radiator. Some theremostat-housing come as a single assembly, so you need to replace them together. If yours is still good, and seals good, you still can re-install it.
Question: My 2008 Ford Explorer is still overheating after changing the whole radiator and water system. What is the cause?
Answer: There could be several reasons for this. This other post may help:
© 2014 Dan Ferrell