5 Symptoms Of A Bad Car Thermostat (& Replacement Cost)

5 Symptoms Of A Bad Car Thermostat (& Replacement Cost)


  • Don’t try to remove your radiator cap or drain coolant right after you run your vehicle since it will be extremely hot and could cause severe burns.

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  • Don’t drive your vehicle if it overheats often since you could cause permanent damage to the engine.

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Thermostat Function

A thermostat consists of two phases: It’s either c

A thermostat consists of two phases: It’s either closed or open.

When the engine is idling, and it is relatively cold, the thermostat is closed, but when the engine starts, and its temperature rises, the temperature of the coolant rises, and so the thermostat opens to let the coolant flow into the radiator, cool down and return to the coolant chamber inside the engine.

This mechanism ensures that the vehicle’s engine operates at its ideal temperature and has no overheating problems. As the thermostat is an essential component for the engine’s well-being, its proper functioning should be ensured.

The thermostat should close and open at the right time; otherwise, your car will develop serious problems.

A step-by-step on how to replace the engine’s thermostat

If you decided to save money on the thermostat replacement cost, here are the step by step on how you can replace it on your own:

  • Make sure that the vehicle is fixed in place using the parking brake
  • Ensure that the car is completely cooled down
  • the thermoset housing is located at the end of the upper radiator hose, where it connects to the engine. 
  • Remove the two large bolts from the thermostat housing using a wrench
  • Pull out the faulty thermostat
  • Install the new thermostat in the correct place. Make sure to install it in the right direction following the vehicle’s manual.
  • Close the thermostat housing and make sure to tighten the two large bolts
  • Fire the engine and repeat any of the mentioned tests before to confirm that the new thermostat was installed correctly.

Can You Drive Without a Thermostat?

A short term solution to a thermostat that is stuck closed is to remove the thermostat entirely.

While it is possible to drive without a thermostat, it is not wise to do so long term. Engines run very poorly before they reach operating temperature.

If you are missing your thermostat, your engine will probably not be able to reach operating temperature (unless you are running your car hard on a race track, and it’s blisteringly hot outside).

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.

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.

How to test the vehicle’s thermostat?

A bad thermostat is easy to find a problem.

There are two ways to test for a bad thermostat: you can check the thermostat while it is connected to the vehicle, or you can pull it out and do a simple test outside the car. 

Before you perform any of the following tests, you need to make sure that the engine is completely cooled down. It is also recommended that you have a friend to rev the engine and to turn it off if it got very hot. 

You first need to locate the engine’s thermostat. If you open the vehicle’s hood, the thermostat housing is located it the end of the upper hose connecting the radiator to the engine. The actual thermostat is sitting inside the thermostat housing box.

When your thermostat is connected to your vehicle, there are two things you need to investigate: 

    • Check the coolant flow: 
      • Open the radiator back.
      • Start the engine and wait till it idles 
      • When you first start the engine, the coolant should not be flowing through the thermostat to the radiator. Simply because it did not reach the required temperature. If the coolant is flowing, this is an indication of a stuck-open thermostat. You can stop the test here.
      • If the coolant was not flowing at the beginning, and it did not flow when the engine got hot, this is an indication of a stuck-closed thermostat.
    • Check the coolant temperature:
      • For this test, you just need to use your hand to monitor the temperature rise in the radiator hoses.
      • Fist start when the vehicle is completely cooled down.
      • Fire the engine and feel the temperature of the hoses. In the beginning, the temperature should be low, and they should feel hotter gradually. 
      • Make sure not to touch the engine when performing this test.
      • Wait about 10 minutes and repeat the previous steps. If the hoses did not get hotter, this indicates that the thermostat is stuck closed, and the coolant is not passing through to the radiator.

For this test, you need to pull the thermostat out of your vehicle, and you will need a kitchen pot, a pair of needle-nose pliers, and a thermometer.

    • Put the kitchen pot on the stove filled with enough water to cover the thermostat.
    • Place the thermostat in the pot without touching the bottom of it. Use a pair of needle-nose pliers for this. 
    • Start heating the water in the pot. 
    • When you see the thermostat valve starts opening, mark the temperature using a thermometer.
    • Keep the thermostat in the pot while the temperature is rising. 
    • Measure the temperature again when the thermostat is fully opened
    • Compare the two temperature values to the vehicle’s specifications. You can find the vehicle’s specifications in the vehicle’s service manual. 

The measured temperature should not differ significantly from the specified temperature in the vehicle’s manual.

If the temperature is completely off the values given in the manual, then you have a bad thermostat, and you need to replace it.

Cabin Temperature Changes

Higher Cabin Temperature / Representation
Higher Cabin Temperature / Representation

Another common symptom of a failing temperature gauge is sudden temperature changes inside the vehicle. If you happen to notice that the temperature inside your cabin suddenly drops and then rise again, this can be a sure shot symptom of a failing thermostat that requires replacement.

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GoMechanic Spares: One Stop Destination For Authentic Spares

Coolant Leaks

Oftentimes when the thermostat fails, it remains in its closed position. As the engine overheats, coolant will overflow out of the thermostat housing. This means that coolant leaking out of your engine could be a sign that your thermostat has gone bad. It can also be a sign that you have a problem with your radiator hoses, the radiator itself, or the seals and gaskets.

2. Low-Temperature Reading and Underheated Engine

A thermostat stuck in the open position constantly pushes coolant into the engine and causes a lower operating temperature. Your temperature gauge will show a needle that barely increases or remains at the lowest setting. This will reduce engine efficiency and increase emisssions over time, as well as accelerate the deterioration of parts.

4. Coolant Leaks Around Thermostat Housing or Under Vehicle

Another indication may be leaking coolant, which can occur when the thermostat does not allow coolant to flow when stuck in the closed position. This can be noticeable in a variety of locations, but most commonly around the thermostat housing. It can eventually cause other coolant hoses to leak as well, often resulting in coolant leaking on the ground under your vehicle.

Thermostat replacement is a fairly inexpensive repair to make to your vehicle, preventing potentially thousands of dollars in engine damage due to excessive heat. If any of the above symptoms sound familiar to you, it may be time to have an expert mechanic diagnose your vehicle.

Strange Noises From The Engine Bay

Noises From The Engine Bay
Noises From The Engine Bay

If temperature changes weren’t enough, you will also experience strange noises from within the engine bay. These noises can arise from within the radiator, or the engine or even both. Resembling a knocking, boiling or gurgling sound, these strange noises can indicate a failed thermostat and a lot many problems in addition to it.

Car Thermostat Replacement Cost

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The good news for those with a faulty thermostat i

The good news for those with a faulty thermostat is that it does not cost a lot of money to have it replaced. Of course, the exact cost will depend on the make and model of your vehicle. But for the average car owner, you can expect to pay between $140 and $300 for a professional mechanic to replace the thermostat in your vehicle for you.

The cost of the actual thermostat unit itself is usually between $20 and $80 but can be more for luxury or sports cars. The cost of the labor will be between $120 and $220 (possibly more if going to a dealership).

Remember that most mechanics will charge around $80 to $110 per hour for their services. The thermostat replacement job should take a mechanic about 1 to 2 hours to complete it. Therefore, you end up paying more money for the labor than you do for the actual thermostat part.

A lot of car owners try to save money by replacing the thermostat themselves. Unless you have some good experience working on cars or your vehicle’s thermostat is in an easy to reach location and you have a good repair manual to guide you, then you better let a professional do the replacement job for you.

After all, there may be another problem with your vehicle as well. You’ll want a professional so they can first diagnose the problem before replacing the thermostat.

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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.

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.

Reader Success Stories

  • Donald Mack

Aug 21, 2018

    Donald Mack Aug 21, 2018

    “The hints and tips on narrowing a stuck closed thermostat were splendidly helpful! I took out the old one on my 2000 Ford Taurus, installed the new one and the cooling system is fine, with 111,000 miles on this car. Thank you!” …” more

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