Bad Thermostat Symptoms

Bad Thermostat Symptoms

If you've ever driven a car, you know how important it is to keep everything running smoothly. Many components go into making your vehicle operate at its best, and one of those is the thermostat.

A car thermostat is responsible for regulating the engine's temperature, which is essential for keeping everything working correctly. However, like any other car part, a thermostat can malfunction and cause problems.

In this article, we'll discuss the common bad thermostat symptoms in a car and their causes. We'll also explore some troubleshooting tips and when it might be time to replace your thermostat.

How Does a Thermostat Work?

A thermostat is a relatively simple device essential to properly functioning your car's engine.

Essentially, a thermostat is a valve that regulates the coolant flow through your car's engine. The thermostat works by opening and closing in response to changes in engine temperature.

When your car's engine is cold, the thermostat is closed, which means that coolant cannot circulate through the engine. This helps the engine warm up more quickly and maintain a consistent temperature.

Once the engine reaches a specific temperature, the thermostat opens, allowing coolant to flow and regulate the temperature. The thermostat works by using a wax pellet inside the cylinder in the center of the thermostat.

As the engine warms up and the temperature of the coolant rises, the wax pellet inside the cylinder begins to melt and expand. This expansion pushes against a piston inside the cylinder, pushing against the thermostat valve.

Coolant can flow through the engine and maintain the proper temperature as the valve is pushed open.

When the engine cools down again, the wax pellet contracts, and the return spring pushes the piston and valve closed again, preventing coolant from circulating until the engine heats up again.

Bad Thermostat Symptoms

Regarding your car's thermostat, it's essential to keep an eye out for any potential problems. If left unchecked, a bad thermostat can cause significant damage to your engine, resulting in costly repairs.

There are several bad thermostat symptoms to look out for in your car.

1. Overheating or Overcooling

One of the most common signs of a bad thermostat is overheating or overcooling. Your car's engine relies on a steady stream of coolant to maintain a safe operating temperature.

And if the thermostat isn't working correctly, it can cause fluctuations in the temperature. This can result in your engine overheating, which can cause severe damage.

2. Leaky Coolant

Another sign of a bad thermostat is coolant leaking. If you notice coolant pooling underneath your car or on your garage floor, it could be a sign that your thermostat isn't properly regulating the coolant flow.

In addition to being messy, coolant leaks can cause your engine to overheat or result in other problems.

3. Temperature Changes

Erratic temperature changes are another potential sign of a bad thermostat. If your car's temperature gauge bounces around or fluctuates rapidly, it could be a sign that the thermostat is sticking or not opening and closing correctly.

This can result in your engine not getting enough coolant, leading to overheating or other issues.

4. Strange Sounds

Strange sounds from your car can also be a symptom of a bad thermostat. If you hear gurgling or bubbling noises from your engine, it could signal that there's air trapped in the cooling system, which can cause a faulty thermostat.

5. Problems With Your Car's Heater

Finally, problems with your car's heater can also indicate a bad thermostat.

If your car's interior isn't getting warm, even when the engine is up to temperature, it could be a sign that the thermostat isn't opening properly. This will prevent the coolant from flowing through the heater core.

A bad thermostat in your car can cause many problems, from overheating and engine damage to poor fuel economy and heater issues.

If you notice any of these signs, it's essential to have your car checked out by a professional mechanic as soon as possible to avoid further damage.

What Causes a Thermostat To Go Bad?

There are several common causes of a bad thermostat. Some of the most common causes include:

1. Overheating

If your car's engine overheats frequently, it can cause damage to the thermostat.

When the motor gets too hot, it can cause the thermostat to warp, preventing it from functioning correctly.

2. Sludge

Over time, sludge and debris can build up in your car's cooling system, including around the thermostat.

This buildup can cause the thermostat to stick, preventing it from opening and closing correctly.

3. Defect

In some cases, a bad thermostat may be due to a manufacturing defect. This can cause the thermostat to fail prematurely or not work at all.

4. Age

Like any car part, thermostats can wear out over time. They may become less responsive as they age, making them more likely to fail.

It would be best if you replaced your car's thermostat every 50,000 to 75,000 miles or as part of your car's regular maintenance schedule.

5. Improper Installation

Sometimes, a thermostat may fail because it was installed incorrectly. This can cause it to become stuck or not function properly.

It's essential to follow the manufacturer's instructions or seek the help of a professional mechanic when installing a new thermostat.

6. Electrical Problems

A damaged thermostat may be due to an electrical problem, such as a blown fuse or faulty wiring.

This can prevent the thermostat from getting the signal to open and close correctly.

7. Low Coolant Levels

If your car's coolant levels are too low, it can cause the thermostat to malfunction.

The thermostat relies on the coolant to regulate the engine temperature. If there isn't enough coolant, the thermostat may not be able to function correctly.

How To Change and Test Your Thermostat

If you suspect your car's thermostat is malfunctioning and needs replacing, it's essential to know that changing it isn't as complicated as it may seem.

The thermostat is typically located near the top of the engine where the top radiator hose joins the engine block.

Once you've found it, you can follow a few simple but essential steps to replace it.

Step 1: 

Place a two-gallon container or a large enough bucket under where you'll be working, as some fluid will escape during the process.

You'll need to return the liquid to the radiator when finished.

Step 2: 

Follow the radiator hose to where the thermostat is located and remove the clamp. You can then pull off the hose when the fluid in the radiator escapes.

Step 3: 

After removing the hose, you'll need to remove the bolts that hold the thermostat housing and the old thermostat in place.

It's also essential to remove the gasket around the housing for the old thermostat (the remaining hole) and scrape off any remaining pieces after removal.

Step 4:

You can now place the new thermostat spring-side down and replace the bolts. After replacing the thermostat and housing, you'll need to reattach the hose and clamp.

Step 5: 

Finally, you can pour the liquid into the radiator's coolant reservoir. It's important to note that different cars may have slight variations in the location and process of replacing the thermostat.

So it's always a good idea to consult your car's manual or a professional mechanic for guidance.

Testing the Thermostat

Once you've replaced the thermostat, it's also essential to test it to ensure it's functioning correctly.

One way to test your car's thermostat is to let the engine warm up and observe the temperature gauge.

If the engine overheats or doesn't reach the appropriate temperature, it could signify that the thermostat is still malfunctioning.

Another way to test the thermostat is to use a thermometer to measure the temperature of the coolant in the radiator while the engine is running. This can help determine if the thermostat is opening and closing correctly.

Changing and testing a thermostat in your car isn't complicated, but following the proper steps is essential to ensure it's well-fixed.

If you're uncomfortable with changing your car's thermostat, it's always a good idea to seek the help of a professional mechanic.


Recognizing bad thermostat symptoms is a crucial skill to have as a thermostat is a simple yet vital component of your car's cooling system that regulates the flow of coolant through the engine to maintain proper temperature.

A malfunctioning thermostat can cause many issues, such as overheating or inefficient engine warming.

Common causes of a bad thermostat include overheating, sludge buildup, defects, and age-related wear and tear. Regular maintenance and inspection of your cooling system can prevent these issues and ensure that your car runs smoothly.

If you need to change or test your thermostat, it is crucial to follow the correct procedures to prevent damage to your engine or injury. The process typically involves locating the thermostat, removing the housing, replacing the thermostat, and refilling the coolant system.