5 Symptoms of a Bad Car Thermostat (& Replacement Cost)

The thermostat ensures your engine will reach the operating temperature as fast as possible. Here's how to tell your thermostat is bad and how much it costs to replace it.

Symptoms Of A Bad Thermostat

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.

With a malfunctioning thermostat, you may experience the engine not operating at an optimal temperature and overheating, which can seriously damage your engine.

In this article you will learn the most common symptoms, location, and the replacement cost of a thermostat. Let’s begin with a quick look at the signs to look for.

The most common symptom of a bad thermostat is an overheating engine or an engine that will never reach the operating temperature. You may also notice a fluctuating temperature on the temperature gauge.

The symptoms give you a warning in advance so that you can take the necessary precautions. Here is a more detailed list of the 5 most common symptoms of a bad thermostat.

Bad Thermostat Symptoms

1. Overheating Engine

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If the thermostat stays closed, the engine’s coolant 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 before it starts overheating again.

Therefore, an unexpected rise in temperature means that you need to see your local mechanic to diagnose and check the thermostat.

2. Temperature Fluctuations

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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 or air in the coolant system, but you should never exclude a thermostat failure from the equation.

3. Heater fluctations

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The coolant warms up the heat inside of your car. Therefore, you might experience temperature fluctuations inside the car heater if the thermostat does not do its job properly to keep a steady temperature in the car’s engine.

If both your temperature gauge and the heat inside the car fluctuate, it is definitely a good time to check the thermostat.

4. Possible Leakages & Steam from engine

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If the thermostat is not working properly, the hot coolant in the engine coolant chamber will find a way to escape because of too high pressure. We all know that hot air rises and that water boils and forms steam when it heats up.

When the coolant heats up, and the pressure gets too high, it can attack the weak points and cause a leak. The coolant is often pressed out of the radiator tank

Leaks are often straightforward to detect because they are visible and often show up in white steam smoke.

5. Rising Temperature and Full Expansion Tank

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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. A rising coolant level can also mean that the car is about to overheat.

Thermostat Function

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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.

Thermostat Location

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The thermostat is often located in a plastic or metal housing near the water pump, connecting to the radiator’s lower hose.

It is most often located on the housing that connects the radiator’s lower hose, but it can be the upper hose in some cars.

Because it is often installed inside a housing, it is in most cases impossible to see with your own eyes without removing it, so to use a repair manual is a good way to find the exact location of your car model.

Thermostat Replacement Cost

The average thermostat replacement cost is between $70 and $450, depending on the car model and labor costs. A thermostat costs around $20 to $50 and the labor cost at a workshop is between $50 and $400.

Some cars have a whole housing with the thermostat integrated, making it impossible to replace the thermostat alone. This can make the part very expensive.

On some cars, the thermostat is placed really badly, requiring some hours of work, while on other cars, you can replace it yourself within 10 minutes.

Check a repair manual to find out how much work it is to replace the thermostat on your car model.