car thermostat

Thermostat Symptoms, Function & Replacement

In Coolant by Magnus Sellén2 Comments

A thermostat controls the coolant flow entering and leaving the engine, and the coolant temperature sensor records the coolant temperature by displaying it on the dashboard. This is valuable information because, without your car’s engine, the engine could very well be saying its goodbyes.

Signs of a Bad Thermostat

The symptoms give you a warning in advance so that you can take the necessary precautions.

1. Overheating Engine

If the thermostat stays closed, the coolant in the engine cannot reach the radiator and therefore cannot cool down. This will cause the engine to overheat – which you can see in your dashboard with an ascending temperature display in the ticker – and you won’t be able to drive for more than 10 minutes. Therefore, an unexpected rise in temperature means that you need to see your local mechanic.

2. Temperature Fluctuations

The thermostat gate must be perfectly timed to ensure that the engine receives the coolant at the perfect temperature. If the timing is not correct, the engine temperature will fluctuate, and the temperature gauge will respond abnormally. This can also be caused by an electrical failure, but you should never exclude a thermostat failure from the equation.

3. Possible Leakages

If the thermostat is not working properly, the hot coolant in the engine coolant chamber will find a way to escape. We all know that hot air rises and that water boils and forms steam when it heats up. When the coolant heats up, it can attack the weak points and cause a leak. These leaks are found in the pipes that connect the radiator to the engine manifold. Leaks are very easy to detect because they are actually visible.

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4. Rising Temperature and Full Radiator Tank

If the thermostat flap remains closed, no coolant can escape to the radiator. The coolant inside heats up and begins to convert to steam, while the coolant inside the radiator remains unchanged. So if you see the temperature gauge rise and the radiator tank is filled with water, you most likely have a problem with the thermostat.

Problems Symptoms Solutions
Bad thermostat Overheating Engine

Temperature Fluctuations

Possible Leakages

Replace Thermostat

Bleed coolant system

Replace Coolant

Thermostat Function


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 that there are no overheating problems. As the thermostat is an essential component for the well-being of the engine, its proper functioning should be ensured. The thermostat should close and open at the right time, otherwise, your car will develop serious problems. Therefore, to know the perfect time for service, you should read the symptoms of thermostat failure.

Solutions for a faulty thermostat

There is not much you can do to solve a thermostat problem, but you can make temporary repairs until you can have the thermostat repaired or replaced by a certified mechanic.

Replace Thermostat/Test It

The only solution for a faulty thermostat is to replace it. On most car models, the thermostat is often quite easy to replace. There is most likely no repair kit for your thermostat on your car, so you will need to replace the entire unit. The thermostat works mechanically, and you can test it by placing it in a bucket of water and heating the water to 90 degrees. When the thermostat is working, you should see that it will open at this temperature (slightly different temperatures depending on the thermostat). If you want to find a thermostat at a good price, I recommend you to check out this one on Amazon: Thermostats

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Bleed Coolant System

In many cases, you can misdiagnose a poor thermostat due to a poorly vented coolant system. Be sure to vent all air from your cooling system.

Repair or Replace

The thermostat is a small part of the engine, so you don’t have to worry too much about emptying your pockets just to have it repaired or replaced. However, it is always advisable to save yourself the trip to the mechanic. Why spend money on repairs when you can save yourself the trouble and just be careful when handling your car? It is not that it is difficult to take care of your car. The routine requires hardly 5 minutes a day and we are sure that everyone is able to manage that.

When it comes to the thermostat, you should clean your car regularly and bring it in for servicing every month so that all the electrical and mechanical components can be rejuvenated. If you fill the radiator and overflow tank regularly, you will never notice the temperature gauge rising. However, if the thermostat fails and the temperature reading rises, you will have no choice but to spend money.

Thermostat Replacement Cost

Your mechanic will not charge too much for labour costs, as the thermostat is easy to find in most cases. You would have to pay $70 to $100 in standard labor costs to have your thermostat replaced or repaired. The repair can take a few hours, which is bearable considering you are safe from future problems. However, when it comes to replacing the thermostat. You must consider other costs.

First, you or your mechanic must find the right thermostat for your vehicle. Finding the right part for a vehicle can be a hectic task and sometimes take days. But sometimes mechanics have a spare part available in their workshop that meets your needs. In this case, you should consider yourself lucky.

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Now, you can imagine what a rare part will cost you, probably a lot of money, but an easily available part will not leave a big dent in your bank account. In reality, it could cost you about $100 to $200.

2 thoughts on “ Thermostat Symptoms, Function & Replacement ”

  1. Dear specialist

    You were talking about closed remained thermostat which makes over hot the engine.

    What about if there is not thermostat or the thermostat remained opened and the liquid circulates always via radiator. Whether this case is dangerous for engine? or if it is, so I have driven always like that, whether it has done any damage to my car engine? When I am driving on descent road in a speed 60 km/h then the temperature gauge ticker goes down slowly.

    Thank you

  2. My 5a engine opens fns at minml gauge yet abv checkedbout all connections what could be wrong

  3. Due to very high demand and a high amount of comments, you might have trouble getting your comment answered by me. If you want to get fast answers from a certified automotive technician you should ask your questions here: Ask A Mechanic

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