Bad Thermostat Symptoms, Causes, and Replacement Cost

A thermostat is the most important part of the engine cooling system. It works according to the engine temperature. It uses to control the coolant flow entering and leaving the engine. The efficient working of the thermostat prevents the engine from overheating and many other problems. As the thermostat goes bad, it generates various symptoms. This article mainly explains the symptoms, cause, and location of the thermostat.

What is Thermostat?

A thermostat is a part of the engine cooling system that closes and opens to permit coolant to flow in and out of the engine. It acts as a valve that activates according to the temperature of the engine.


The thermostat activates as the engine temperature increases and allows coolant to flow from the radiator to the engine.

Symptoms of Bad Thermostat

As the thermostat goes bad, it generates one of the below-given symptoms:

  • Engine Overheating
  • Temperature Fluctuations
  • Heater Fluctuation
  • Full Expansion Tank
  • Poor Engine Performance
  • Rumbling Noises
  • Coolant Leaking

1) Engine Overheating

The engine overheating is one of the main signs of a bad car thermostat. The thermostat works as a valve that allows the coolant to flow from the radiator to the engine and vice versa. When the engine temperature increases, the thermostat opens and allows the coolant to flow throughout the engine.  

engine overheating

As the vehicle thermostat fails, the coolant cannot flow to the engine, and the vehicle will eventually overheat. Therefore, when your vehicle overheats, immediately check your thermostat, and if it goes bad, then fix it.

Read More: Engine Overheating Symptoms

2) Temperature Fluctuations

The thermostat must be opened and closed according to the engine’s requirements so that engine can properly maintain its temperature. The perfect timed opening and closing of the thermostat ensures the proper supply of coolant to the engine and maintains the normal temperature of the engine.  

If the timing is wrong, you will see a fluctuation in the engine temperature, and the temperature gauge will respond abnormally. The temperature gauge is installed on the vehicle dashboard.

Temperature Gauge, signs of bad thermostat

You may also face temperature fluctuation issues due to power or air faults in the cooling system.

3) Heater Fluctuation

The car heater takes heat from the hot coolant of the radiator. As the heater extracts heat from the coolant, it transfers this heat inside the car. Therefore, when the thermostat is not working properly to keep the car’s engine at a constant temperature, the heater will not work properly, and you may face temperature fluctuation issues.  

Car Heater, signs of bad car thermostat

When the thermometer and the heat in the car are fluctuating, it’s the best time to check the thermostat.

Temperature changes that do not reflect current HVAC settings usually indicate a problem with your car thermostat.

Read More: Why Car Heater Blowing Cold Air?

4) Full Expansion Tank

In the case of a stuck closed thermostat, the coolant can’t flow from the radiator to the engine. In such a condition, the coolant in the radiator remains the same, but the coolant inside heats up and begins to turn into vapor.

Expansion Tank

Therefore, if the temperature gauge goes up and the radiator reservoir is filled with coolant, it means you may have a faulty thermostat.

5) Rumbling Noises

The rumbling noise from the vehicle is one of the obvious signs of a stuck thermostat. The vehicle may generate rumbling noise due to a fault in the radiator, the engine, or both.

Rumbling Noise

So, if you are hearing this kind of noise or noticing the old car thermostat symptoms, there is a high chance that you have a bad thermostat.

6) Coolant Leaks

As the thermostat fails, it gets stuck in the off position. This means that when the thermostat is stuck in the closed position, the coolant will not flow to the engine. This causes the coolant to leak out of the thermostat casing.

Coolant Leaks

If it is stuck in the closed position and you don’t fix it, then ultimately, your coolant hose will be leaked. The easiest method to check for coolant leaks is to look under the vehicle. If the vehicle is leaking green or red liquid and the underlying surface is dirty, there is definitely a coolant leak.

Read More: Coolant Leak Symptoms and Causes

7) Poor Engine Performance 

The thermostat is a crucial component for maintaining the engine’s normal temperature, directly impacting its performance. A malfunctioning thermostat can lead to various issues that affect the engine’s efficiency.

Poor Engine Performance

When the thermostat fails, the engine may experience performance problems, including decreased fuel economy. The engine will have to work harder to compensate for temperature imbalances, resulting in reduced fuel efficiency.

Additionally, a faulty thermostat can cause the engine to overheat or fail to reach the desired operating temperature. This not only puts the engine at risk of damage but also leads to increased emissions, contributing to environmental pollution.

Ensuring the proper functioning of the thermostat is essential for maintaining engine performance, fuel efficiency, and reducing emissions.

Causes of a Bad Thermostat

The following are the most common causes of a bad thermostat:

  • Engine Overheating
  • Sludge
  • Manufacturing Defect
  • Poor Maintenance
  • Age

1) Engine Overheating

The thermostat works according to the temperature of the engine. It opens and closes according to the engine heat.

If your engine overheats due to some reason, the thermostat may fail. Thermostat parts are manufactured to work at normal operating temperatures of the engine, while excessive overheating can damage these parts.

2) Sludge

With time, the coolant contaminates and begins to fail. The major problem is that the coolant condenses and converts into thick, sludge-like material with time. This sludge can enter the thermostat and restrict or completely block the flow.

The contaminated coolant can also prevent the thermostat from giving correct readings, causing delays in closing and opening, which may cause engine overheating or hypothermia.

3) Manufacturing Defect

Like many other car parts, thermostats are mass-produced in factories. Before the thermostat goes on sale, the manufacturer inspects its efficient working.

Human factors play an important role in controlling the behavior of thermostats. During the inspection process, the manufacturer may miss some defects that may cause of a bad thermostat.

4) Maintenance

The proper repair and maintenance of the thermostats are very important for their efficient working. If you don’t maintain the thermostat properly, it will not work properly and may lead to different issues.

5) Age

Age is another common cause of failure. The thermostat heats up and cools down all the time, and internal parts can wear out over time. This usually happens slowly, but the temperature at which the thermostat opens will start to rise until, one day, total overheating occurs.

Thermostat Replacement Cost

The thermostat replacement cost varies according to your living area, type of brand, and car model. The average replacement cost of the thermostat is between $60 and $470. In this cost, the labor cost is from $40 to $380, while the thermostat cost is between $20 and $90.

Thermostat Location

The thermostat is located in a metal or plastic casing near the water pump. In some vehicle models, it is located near the lower hose of the radiator. It is recommended to check your vehicle manual to quickly find the exact location of your thermostat.  

How to Test a Bad Thermostat

Testing the car thermostat doesn’t require extensive mechanical knowledge and can be done through a simple visual examination. Here’s a step-by-step guide:

  1. Ensure the engine is cold before starting the test. Locate the radiator cap, typically found near the front of the car under the hood. Remember, never open the radiator cap while the engine has high temperature.
  2. With the engine off, remove the radiator cap and observe the inside of the neck. The coolant should not be flowing into the radiator while the engine is cool. If you notice coolant flowing, it indicates that your thermostat is stuck open.
  3. Start the engine and let it run. While the engine is warming up, pay attention to the coolant flow. If you see no coolant movement despite the rising temperature on the dashboard, it suggests that the thermostat might be stuck closed.
  4. Whether the thermostat is stuck open or closed, it is important to address the issue to prevent further engine damage.

FAQ Section

Can you drive a car with a bad thermostat?

Yes, you can drive but it is not suggested to drive a car with a faulty thermostat. Continuing to operate a car with a faulty thermostat can lead to potential engine damage over time. It is crucial to remain vigilant for signs indicating a malfunctioning thermostat to ensure safe driving conditions. When you notice signs of a bad thermostat, you should immediately contact a professional mechanic.

What is the function of the car thermostat?

The thermostat in a vehicle acts as a valve that opens and closes based on temperature changes. It serves to separate the engine from the radiator until the engine reaches a specific minimum temperature. Without a functioning thermostat, your engine may consistently dissipate heat to the radiator, resulting in a longer warm-up time.

Does thermostat control AC?

Yes, a vehicle air conditioning system (AC) contains a thermostat. This component allows you to set the desired temperature for your living space and determines when and for how long the system operates.

Does a bad thermostat affect AC performance?

A malfunctioning thermostat can result in inadequate cooling or heating of a car interior through the AC system. Monitoring the thermostat is crucial for assessing the overall health of the air conditioning system.

What happens if car thermostat is stuck closed?

Thermostat failure often manifests through inconsistent operating temperatures. If the thermostat remains closed, it can cause your engine temperatures to rise uncontrollably. Conversely, if the thermostat is stuck open, your engine may run at a lower temperature, affecting its performance.

What are the symptoms of stuck closed thermostat?

  • Decreased engine performance
  • Increased pressure in the cooling system
  • Loss of heat from vents
  • High coolant temperature
  • Engine overheating

Can a bad thermostat cause my car not to start?

No, a faulty thermostat usually does not directly cause a vehicle not to start. The thermostat’s main function is to control engine temperature and has no direct impact on the starting process of a car.

What causes a thermostat to go bad in a car?

  • Engine overheating
  • Sludge
  • Manufacturing defect
  • Poor maintenance
  • Wear and tear
  • Electrical failure
  • Stuck valve
Read More

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