What Should I Do If My Car Thermostat Starts to Fail? (2023)

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A car's thermostat is a simple and straightforward device. But if it fails, it could lead to serious problems. Learn more about diagnosing the signs of a failing car thermostat.

Car Thermostat Overview

A car’s thermostat regulates the flow of coolant through the engine and is an essential part in the operation of your vehicle. In most cases, an overheating or no-heat condition in your vehicle is due to a faulty thermostat. And since the T-stat price is only about $8, it makes more sense to replace it than to spend hours diagnosing the problem. If that doesn’t fix it, you’ve only spent about two hours learning how to replace a thermostat.

Symptoms of a Failing Car Thermostat

Fortunately, a thermostat failure is one of the many car problems your can diagnose and fix yourself. Here are the signs your car thermostat is failing:

  • The temperature gauge reads high and the engine overheats.
  • The temperature changes erratically.
  • The vehicle’s coolant leaks around the thermostat or under the vehicle.

Engine Overheating

Here’s how thermostats work. The manufacturers inject a mixture of ground-up brass and wax into a copper cup called a pellet. Then they slip a highly-polished metal rod through a rubber “O-ring” gasket and into the wax. The pellet is sealed with a crimp ring.

(Video) How to Tell Car Thermostat Is Bad (Symptoms of Stuck Open Stuck Closed and Overheating)

Next, a metal “skirt” is welded around the pellet. When the engine is cold, a spring forces the skirt up against a seat (just like a closed faucet), stopping the flow of coolant. As the engine heats up, the wax melts and expands.

Expansion pressure builds to the point where the wax tries to “spit out” the metal rod. But the rod can’t go anywhere. It’s attached to a “bridge” on the other side of the thermostat.

Eventually the pellet itself moves, overcoming the spring’s pressure. So the thermostat opens and allows coolant to flow. The entire system works well until the metal rod corrodes. That corroded rod damages the rubber seal and the wax leaks out. Once that happens, the thermostat stops opening, coolant stops flowing and your engine overheats. The result can be catastrophic engine failure costing several thousand dollars.

Erratic Temperature Changes

Keep an eye on your dashboard’d temperature gauge. If the needle spikes and drops, a thermostat failure may be to blame.


Coolant Leaking

Check the thermostat housing for leaked fluids. The coolant helps to keep your engine functioning at the right temperature, and the lack of fluid can affect its performance.

How to Test a Car Thermostat

Before removing the thermostat from your vehicle, check the coolant flow and temperature.

Remove the radiator cap and idle the engine. If the coolant flows right away, your thermostat is stuck open. If it is not flowing, wait until your engine warms up. The coolant should warm to the right temperature and begin to flow after 10 to 20 minutes. If it does not start to flow, but the temperature gauge on your dashboard rises, your thermostat is stuck closed. If your coolant flows normally, there may be another cause of your overheating engine.

To check the temperature of your coolant, start with a cool radiator and engine. Idle your engine and use a thermometer to read the engine block or cylinder head temperature. Then, check the temperature of the upper radiator hose. Wait five minutes and test again. Complete the test a total of three times. If the temperature does not rise by much, your thermostat is stuck open. If the radiator hose remains about the same temperature, but the dashboard gauge rises dramatically, your thermostat is stuck closed. If the upper radiator hose rises to about the same temperature as the engine block, your thermostat is allowing the coolant to flow properly. You may have a different problem causing cooling issues.

(Video) How to Test if Your Car's Thermostat is Working (Without Removing It!!)

To test the thermostat outside of the car, place it in a pot of water on your kitchen stove. Do not let the thermostat touch the bottom of the pot. Use a thermometer to see the temperature at which the thermostat opens. Note the temperature when it begins to open and when it finished opening completely. Remove it from the pot. Observe as it cools to see that is closes gradually. Compare your notes to your owner’s manual.

Replacing the Thermostat

If your thermostat fails, don’t just replace it and top off the coolant reservoir. It’s sending you a message of a more serious problem — the corrosion inhibitors in your coolant also failed. So flush the system and add fresh coolant any time you replace the thermostat.

And don’t try to save a few dollars by buying a cheap thermostat (about $5). Premium thermostatsare built to resist corrosion. The difference in price is small compared with the potential engine damage. Some manufacturers even offer “fail-safe” models that fail in the fully open position to prevent engine overheating.


Is it OK to drive with a bad thermostat? ›

Can I Still Drive with a Bad Thermostat? The easy answer to this question is no. While your car may be physically able to move and get you from Point A to Point B, you will want to refrain from operating your vehicle. This can lead to more parts of your vehicle being damaged, especially if the engine is overheating.

What happens when car thermostat fails? ›

As the engine heats up, the thermostat gradually opens, allowing coolant to reach the radiator. Whenever it begins to fail, the engine overheats and, if the problem is left unchecked, leads to your engine boiling over. This can lead to costly repairs.

How long can I drive with a bad thermostat? ›

If the thermostat is locked in the “open” position, you can drive the car for as long as you want, assuming you want no heat. In modern cars, the proper engine temperature does affect the fuel mileage through the ECU (computer), but it really does no long term damage.

What causes a car thermostat to fail? ›

Thermostat failure can be caused by contaminated fluid or simply an old thermostat with weakened springs. Contamination happens when incompatible coolants are mixed, old coolant is not changed as recommended or particulate matter from gaskets or elsewhere breaks off.

What is the average life of a car thermostat? ›

You may never need to worry about replacing your car's thermostat because they last 10 years on average. This being said, if you plan to keep your car for a long time, or if you drive an older automobile, there will come a time when you need to replace the thermostat.

How much does it cost to replace a thermostat in a car? ›

But for the average car owner, a professional mechanic will charge between $140 and $300 to replace the Thermostat in your car. The thermostat unit usually costs between $20 and $80, but a luxury or sports car can cost more. In addition, the labor will cost between $120 and $220.

How do I know if my car needs a new thermostat? ›

Signs My Car's Thermostat Is Going Bad or Has Failed Completely
  1. Odd Temperature Gauge Readings. Your temperature gauge should register cold when you first start your car and slowly build up to normal as the engine warms up. ...
  2. Air Temperature Changes. ...
  3. Coolant Leaks. ...
  4. Strange Sounds. ...
  5. Heater Problems.
May 15, 2021

How much does it cost to fix a broken thermostat? ›

Most homeowners spend between $111 and $308 to repair a thermostat. Exact costs range from about $50 to $500, with a national average of $209. The price may will depend on factors like the model you own and whether it has a warranty.

Is it easy to fix a thermostat in a car? ›

Replacing a car's thermostat (or T-stat) is an easy and inexpensive repair, and in most cases will cure an overheating or no-heat problem, sparing the time and expense needed for expert diagnostics.

What are the signs of a bad thermostat? ›

Thermostat failure symptoms that you should know
  • High temperature readings. The main symptoms of a failing thermostat are related to engine overheating. ...
  • Erratic changes in temperature. ...
  • Leaking coolant. ...
  • Increased fuel consumption. ...
  • Unusual noises.
Sep 15, 2022

Can your car overheat with a bad thermostat? ›

Faulty Thermostat

A faulty thermostat might remain closed even when the engine is hot, which can quickly lead to overheating.

How do you tell if my thermostat is bad in my car? ›

Here are the signs your car thermostat is failing:
  1. The temperature gauge reads high and the engine overheats.
  2. The temperature changes erratically.
  3. The vehicle's coolant leaks around the thermostat or under the vehicle.
May 5, 2022

Can you fix a faulty thermostat? ›

Luckily, they're also easy to fix. If cleaning the thermostat or tightening its connections doesn't fix the problem, consider replacing the thermostat. We recommend switching to a smart digital thermostat. Digital thermostats help keep your home more comfortable and even reduce your heating or cooling bills.

What happens when your thermostat goes out? ›

If the thermostat goes bad, it can cause the HVAC system to switch on and off irregularly. This results in places closer to the HVAC system getting more cool/hot air, with rooms furthest away getting barely any. The Interface Won't Respond.

What kills thermostat? ›

Mainly age and corrosion are the culprits. Failure of parts falls closely in line with the aforementioned reasons. But, WHAT causes this? Heat, lack of coolant in the mix/ratio of coolant vs water and possibly a defective manufacture of thermostat.

How often do car thermostats fail? ›

There is not a set time for the car radiator to stop working. However, most car experts recommend car thermostat replacement after 10 years.

Does a faulty thermostat affect performance? ›

If your thermostat fails, the engine will not operate within its ideal temperature range. In turn, this will affect the -performance of your vehicle.

How long does it take a mechanic to replace a thermostat? ›

An average thermostat replacement takes around 1 hour, with some taking as little as 30 minutes. More problematic replacements can take a little longer where there are issues accessing an awkwardly positioned part or if it's become seriously stuck in place.

How many hours does it take to replace a thermostat? ›

Installation typically takes about 1 hour. Once the new thermostat has been installed, your technician will help you program it if you'd like.

Can I replace my thermostat myself? ›

This is usually a simple straightforward job that can be completed by a DIYer. Some systems may need a new wire or set of wires run from the furnace to the thermostat, which can be challenging if you don't have experience running wires.

Do you need to replace coolant when replacing thermostat? ›

The cooling system is recommended to be flushed out anytime the thermostat is replaced if the coolant has not been changed or flushed recently. A thermostat does not always require the coolant to be flushed out and would be an additional charge for parts and labor.

Can I drive without a thermostat? ›

If you drive without a thermostat, your engine will never reach its optimal operating temperature and your heater may blow cold air. If you live in a warm climate, this should not be a problem.

Can a faulty thermostat cause a fire? ›

The programmable thermostats constantly charge the backup AA batteries used to power the thermostat's clock. This can cause the batteries to leak, resulting in a fire hazard.

How do I check my car thermostat? ›

Start your car's engine and allow it to idle. Look through the radiator filler neck to see if the coolant flows. At this time, it should not be flowing as your car has not reached the operating temperature to cause the thermostat to open. If you find the coolant is flowing, it means the thermostat valve is open.

Can a bad thermostat affect car AC? ›

The thermostat should not affect the A/C compressor blowing cool air. The thermostat regulates the flow of the coolant in your car's engine and controls when the coolant flows in and out of the motor from the radiator by opening and closing at specific temperatures.

Can a bad thermostat cause engine problems? ›

If the thermostat becomes stuck in the open position, there is continuous flow of coolant into the radiator causing the engine to run cold. Overcooled engines run inefficiently, which leads to increased fuel consumption and higher emission levels and engine parts enduring more wear.

How do you check if a thermostat is working? ›

Standard thermostats: Enlist a friend or family member to assist, Position one person by the thermostat and one person by the furnace. Slowly turn the thermostat from off to heat and increase the temperature. Both the thermostat and furnace should make a sound.

How do you temporarily fix an overheated car? ›

5 steps to take when you have an engine overheating:
  1. Turn on the heater. It sounds counterintuitive, but Reina recommends turning on the heater. ...
  2. Sit tight. If you don't have roadside assistance, be patient; it will take at least 15 minutes for the engine to cool. ...
  3. Add coolant. ...
  4. Drive to a service station.

Does a bad thermostat turn on check engine light? ›

Your car's thermostat is a device that opens and closes to regulate the temperature of your engine. If this device is stuck open or stuck closed, either could trigger the “Check Engine Light” light to come on. What to look for - Thermostat Failure: If the thermostat is stuck closed, the vehicle will start to overheat.

Can I still drive my car if it overheats? ›

If your car is overheating, you should definitely not drive it. This could be due to several factors, including low coolant levels or a faulty cooling system. Driving an overheated car puts additional strain on the engine, leading to further damage and costly repairs down the road.

Can a bad thermostat cause AC not to cool? ›

A faulty thermostat may be to blame if the air conditioner short cycles and runs almost nonstop, the living space seems warmer than your desired temperature setting, or you suddenly experience a cooling system failure. Learning how to troubleshoot these issues can help you avoid an unnecessary and costly service call.

Does a bad thermostat make noise? ›

The noise can be heard as a boiling sound, gurgling sound, or knocking sound. Therefore, when you start hearing such noises or experiencing the above bad car thermostat symptoms, you most probably have a bad thermostat engine.

Can I drive home if my car is overheating? ›

If you suspect your car is overheating, the best thing to do is stop somewhere safe and let it cool before trying to drive again. However, if your car has already overheated, you'll have to drain all of the engine's coolant to prevent further damage.

How much does it cost to fix a bad thermostat? ›

With Angi and HomeAdvisor reporting a typical cost range between $113 and $255 nationally, it's a small repair job that can save a lot of money in the long run.

Is it bad to drive with thermostat stuck open? ›

If it is stuck open, it will take longer for the engine to heat up and for the heater to put out hot air. No big deal. if it is stuck closed, the coolant won't circulate through the radiator. The engine will overheat and you can seriously damage many engine parts.

What does a bad thermostat sound like? ›

The lack of coolant flow due to a failing thermostat could also cause rumbling noises as your car engine begins to overheat. You may hear knocking, boiling, rumbling or gurgling sounds from your engine or your radiator. In this case, contact a mechanic immediately.

Can a bad car thermostat affect AC? ›

The thermostat should not affect the A/C compressor blowing cool air. The thermostat regulates the flow of the coolant in your car's engine and controls when the coolant flows in and out of the motor from the radiator by opening and closing at specific temperatures.

How can I temporarily fix my overheated car? ›

5 steps to take when you have an engine overheating:
  1. Turn on the heater. It sounds counterintuitive, but Reina recommends turning on the heater. ...
  2. Sit tight. If you don't have roadside assistance, be patient; it will take at least 15 minutes for the engine to cool. ...
  3. Add coolant. ...
  4. Drive to a service station.

How long should I let my car sit if it overheats? ›

Wait until the engine completely cools — at least 30 minutes. You might be able to speed up the cooling process if you can pop the hood with a latch located inside the car cabin. But, do not touch or attempt to open the hood until the engine is completely cool. Check the coolant/antifreeze reservoir.

How can I cool down my car engine while driving? ›

Pull over and open the hood. This releases the heat and lets air circulate through the hot engine. Just be careful when opening the hood—hot steam may come blasting out.


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