Top 5 Bad Thermostat Symptoms (Replacement Cost)

Thermostat controls the temperature inside the engine of a vehicle; so a bad thermostat is most likely going to cause the engine to overheat. Not just overheating, a faulty thermostat can cause different anomalies that could make the affected car “un-driveable.”

When your car’s thermostat is bad, it exhibits certain symptoms, and it is advisable to fix the bad thermostat as early as possible. To help you detect when you’ve got an ad thermostat, this article explains the top common bad thermostat symptoms applicable to almost all vehicles that have a combustion chamber

When you see any of these signs, you should first troubleshoot the vehicle to confirm that the signs are triggered by a bad/failing thermostat. This preliminary check/troubleshooting is advisable because some of the signs of a bad thermostat are equally symptoms of a bad head gasket and other parts too.

What Does a Thermostat Do?

automotive cooling system

The thermostat does a pretty good job to keep the engine cool and functioning at good efficiency. Majorly, it helps to control the temperature (heat) inside the engine.

When the temperature is too high inside the engine, the thermostat opens up to allow coolant flow into the engine to keep it at the regular operational temperature.

Thermostats are quite small, but they play a huge role in every ICE vehicle. When the thermostat is fully functioning, it reduces the chances of overheating.

However, for pretty many reasons, thermostats can go bad – physical damage or clog. There are two main issues with thermostats; they either get stuck open or stuck closed.

Either stuck open or stuck close, a bad/faulty thermostat would result in other costlier engine damages if not fixed on time. Interestingly, this article also analyzes the cost of fixing a bad thermostat; but first, let’s discuss the symptoms of a bad thermostat.

Top 5 Bad Thermostat Symptoms

1. Overheating

As expected, right? Yes, any damage to a component that makes up the cooling system of ICE vehicles would lead to overheating. The thermostat is part of the cooling system, as such, its failure can cause overheating to occur. The reason is quite apparent.

When a thermostat is bad (stuck close), it restricts coolant from getting into the engine. This makes the engine heat up pretty much – overheat – and this can lead to the damage of many other components inside the engine. If overheating is allowed to linger, you may end up needing a complete engine replacement.

So, if your car’s engine frequently overheats, one of the things to check out is the thermostat. A stuck open thermostat may not cause overheating, anyways; overheating usually ensures when the thermostat is stuck closed and coolant cannot get into the engine to keep it at operational temperature.

2. Overcooling

Well, just as you guessed, this is the opposite of “Overheating.” Overcooling is not good for an engine – when an engine is too cold, it causes the engine to run inefficiently and also utilize more fuel. So, just as overheating isn’t good, overcooling isn’t good too.

As said earlier, a bad thermostat either gets stuck opened or stuck closed. When it’s stuck closed, it causes overheating, and when it is stuck open, it caused “Overcooling.” This happens because the coolant would continuously flow – unregulated – into the engine (even when there’s no need for that).

Because the thermostat is open and coolant keeps getting into the engine, the engine gets extremely cool and begins to run sluggishly. Yes, if the engine temperature is below operational temperature, your vehicle would use more fuel and oil. So to say, overcooling causes a significant decline in fuel economy.

3. Coolant Finishes Rapidly

If you find out that the coolant liquid constantly finishes within a short time with no trace of leaks and the engine doesn’t overheat too, then you probably have a stuck open thermostat to deal with.

A stuck open thermostat would allow the coolant to uncontrollably flow into the engine; the flow continues until there’s no coolant liquid in the reservoir and radiator. So, you’d find yourself refilling coolant all the time.

But, if you could detect that the coolant leaks out from somewhere, then the thermostat is most likely not the culprit – but the leaky spots.

4. High Fuel Usage

Surprised? You don’t need to be. A bad thermostat can actually cause high fuel usage. How is this possible? This usually happens when the thermostat is stuck open – allowing the coolant to flow without regulation.

With constant coolant flow into the engine, the engine would be very cool and utilize more fuel/oil to keep running. This way, you’d notice that your car uses more fuel than usual. A stuck closed thermostat won’t cause this sign, anyways.

5. Heater / Temperature Fluctuations

The thermostat is designed to open when the temperature level inside the engine increases above operational temperature – and close back after a reasonable amount of coolant has gotten into the engine. When the thermostat opens up, hot steams would escape into the engine bay and disperse.

Now, when the thermostat is bad, it would cause temperature irregularities; thereby causing the temperature gauge inside the car’s dashboard to give erratic readings.

Also, a bad thermostat can cause the heater in a vehicle to malfunction; this is because the heater feature is designed to work with the thermostat.

How To Check For a Bad Thermostat

The thermostat is placed in different locations, in different vehicles. Trying to access the thermostat may require disassembling some components that make up the cooling system of the vehicle. To see the thermostat, you may need to follow the repair manual for your car model.

For the sake of this, it is advisable to allow a professional mechanic physically inspect your car for a possible bad thermostat problem. However, when you start noticing any of the signs explained above, it is most likely the thermostat in your engine is gone bad.

Replacing a bad thermostat isn’t something you should do individually, especially if the thermostat in your vehicle is hidden in the radiator housing.

Thermostat Replacement Cost

The average cost of buying a thermostat is between $20 – $80, depending on the car model and make. For some special/luxury vehicles, the cost is above $100. Labor costs can fall from $30 – $50, depending on the workshop where you went to get the service done.

The average cost of replacing a thermostat is between $100 to $400 based on variable factors. Also, if the bad thermostat had caused other damages, the cost of repairing those damaged components would be calculated differently. The final price varies from workshop to workshop.

Frequently Asked Questions

How Do u Know If Your Thermostat Is Going Bad?

When the coolant liquid is always drained out, or when the coolant liquid isn’t reducing and the engine frequently overheats – those are signs that the thermostat in your car is about to go bad.

Can Bad Thermostat Be Repaired?

It is not advisable to repair a bad thermostat – it can’t be repaired anyways. The best way to fix a bad thermostat is to get a new one. Interestingly, the unit itself isn’t too expensive.

What is the Cost Of Thermostat Replacement?

The average cost of replacing a thermostat is between $80 – $400, depending on your vehicle model and the type of thermostat installed in the vehicle. This price includes the labor cost.

Where Is The Thermostat Located?

It is usually installed around the water pump – in a dedicated housing. To find the exact location of the thermostat in your vehicle, you need to check the repair manual for your vehicle model.


In conclusion, this article explains the common bad thermostat symptoms you would most likely experience when the thermostat in your car starts going bad. It is important to fix the thermostat as early as possible to avoid incurring other expensive damages.

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