Learn About Bad Car Thermostat Symptoms & Signs

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bad car thermostat symptoms

Maybe your engine is overheating or maybe it’s not running as smoothly as it was before. These bad car thermostat symptoms are among the most common with a faulty or worn-out thermostat.

So what are the most common faulty thermostat symptoms? As it can either get stuck open or closed, the thermostat can cause engine low performance, extra fuel consumption, overheating and fluctuations in temperature. 

Why does this matter?

What is a car thermostat?

The car thermostat is referring to a wax thermostatic element and the assembly that is created with it. The wax thermostatic element can transform the heat radiated by a source into a mechanical movement. It can do this because of the thermal expansion possessed by these waxes when they are melting.

The principle used by the wax thermostatic element is not only used in the automotive industry but also in other industrial sectors.

This wax that is used to build a thermostat device, it’s a special one, created especially for this scope. Usually the requested function is determining how much in size the wax should melt or not.

If you want to learn a bit more about different wax elements and how they are defined, I will leave you a link.

I will leave you also an illustrative video, simulating the behavior of a thermostat:

What are the symptoms of a bad car thermostat?

The thermostat is a key element for the proper functioning of an engine. Either if it’s getting stuck open or closed it will be bad for the car’s engine.

When it’s stuck closed, the coolant will not flow to the radiator and thus the engine will start overheating. When it’s stuck open, it will flow always to the radiator reducing the optimal engine temperature. This will cause poor performance, extra fuel consumption and increased pollution.

So what are the most common symptoms of a bad car thermostat?

1. Poor engine performance

Like we talked about, this will be the most obvious one if you know your car. The issues can happen when the thermostat will remain open all the time. This might be also easy to see it on the dashboard, as the temperature indicator may be below the normal indicating state.

Other issues can cause this problem, but it’s worth taking a look at the car thermostat, as it’s very cheap to replace.

2. Temperature indicator light ON

This will be visible and can happen in a very short time if the thermostat is stuck closed. As the flow of coolant will not go to the radiator anymore, the engine’s temperature is rapidly increasing.

As the car has a triggering system, in case of an emergency like this, sound and light will pop up on your dashboard. Of course, again, in this case, other issues like a bad water pump can also cause this, it’s wise to check every part.

3. Temperature fluctuations

A bad or faulty thermostat can accompany different temperature fluctuations in a car. Because the thermostat will function irregularly, the coolant’s flow will be not constant or ideal. This will cause different reading errors, which will lead to abnormal behavior.

This can be a great sign that the thermostat is not doing its job correctly. It will need more investigations of course, but have this bad car thermostat symptom in mind if you encounter any of these signs.

What does the thermostat do in a car?

When we’re referring to the thermostat in a car, we’re thinking about the car’s cooling system. For an internal combustion engine, the optimum temperature for running smooth is around 95 degrees Celsius. The thermostat helps in achieving this optimum temperature by adjusting the coolant’s flow to the car’s radiator.

They place the element perceiving the temperature and the control valve in the same location, creating a simple flow control device.

When you start your engine, the thermostat is closed to allow a fast engine warm-up. Modern engines run at high temperatures to reduce the emissions and work more efficiently.

When the thermostat is closed, there is no coolant flow to the radiator, and it’s directed to the engine only. Depending on the car thermostat properties, this one has a nominal temperature where it will open. As soon as the engine is reaching the optimal temperature, the thermostat will open, letting coolant flow through the cooling system.

The therm0stat opening and closing of the valve is controlled further by the coolant temperature, to keep that optimal engine temperature.

Well, anyway, this all leads me to

How long does a car thermostat last?

It’s a tricky question, but the answer can be straightforward. If you take good care of your car and your can’s cooling system, you might not have to change it in a very long time.

The car’s coolant should be replaced according to the owner’s manual. This way you are protecting the cooling system, but also the car thermostat.

Also, if you’re having some servicing actions in an area where the thermostat is located or you need to take it out, it might be a good idea to replace it.

What happens when the thermostat is stuck closed?

Like we talked at the beginning of this article, the car thermostat will be in a closed position when the vehicle is turned on. This means that the coolant will not flow through the radiator to cool down and it will flow only through the engine.

While the engine is heating, the coolant is still not flowing to the radiator. When this happens, your engine is overheating and not function accordingly.

The temperature indicator in your dashboard will increase over the optimum temperature and at some point it will trigger an alert. An engine must be cooled down, otherwise, it can be damaged and the repair costs will not be so appealing to you.

You might wonder:

Can you test the car thermostat without removing it?

Usually, this is possible and we will talk about two ways to do it:

1. Checking the coolant flow in the radiator:

  • if you drove your car, let it cool for a while, as the next action is dangerous to perform within a heated system;
  • carefully open the radiator cap and rest it aside;
  • turn on the car and let it with the engine running for at least 10 minutes;
  • always check how the temperature is rising in your dash indicator;
  • when it’s near the optimal engine temperature, the thermostat shall open to let flow pass through the radiator;
  • try to look carefully, but not too close as it can be dangerous for you;
  • if you can see a flow of liquid running (maybe try to use a flashlight) then it means that the thermostat went from closed to open and it’s functioning correctly;
  • don’t forget to put back the radiator cap;

2. Use a laser thermometer

  • steps will be as above about starting the engine and letting it idle;
  • try to measure the temperature of the black hoses that are entering and exiting the radiator;
  • the one that’s going to the engine should have a lower temperature than the other one that’s entering the radiator;

The second option is more difficult to perform, so I suggest sticking to the first want if you are trying to see how you can test the car thermostat without dismounting it.


Usually the car thermostat is a pretty life-lasting part, but there are cases where bad car thermostat symptoms arise and it forces you to replace it. It’s not that expensive and also it’s not that a difficult task. The important thing is to get a good thermostat, suitable for your car as it’s an important piece in the good functioning of your motor.




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