How Does A Car Thermostat Work?

If your car is running hot, or making strange noises when you turn on the air conditioning, then it could be due to a malfunctioning thermostat. But before you take it in for repairs, it’s important to understand how does a car thermostat work and whether that fix would cost more than simply replacing the part itself.

In this blog post, we’ll explain exactly why cars need them as well as how they affect different systems in a vehicle so that you can make an informed decision about maintaining the health of your vehicle.


Acting like a valve, the thermostat regulates the flow of coolant between the engine and the radiator based on the engine’s temperature. By keeping the engine isolated from the radiator until it reaches a minimum temperature, the thermostat ensures that the engine warms up as quickly as possible. Without this mechanism, the engine would only constantly lose heat, resulting in longer warm-up times.

How Does The Thermostat Know When to Open and Close?

How-Does-A-Car-Thermostat-Work (1)
How-Does-A-Car-Thermostat-Work (1)

The thermostat is equipped with a wax-filled cylinder on its side. As the engine temperature rises, the wax expands and gently pushes on a rod that opens the thermostat, allowing coolant to flow. And as the engine temperature drops, the wax slowly begins to harden, signaling the thermostat to close. With this built-in mechanism, you can rest easy knowing that your car is being maintained at the ideal temperature for optimal performance.

What Are The Symptoms of Bad Thermostat?

A bad thermostat can lead to a variety of problems, including:

Overheating And Overcooling

One of the most common symptoms of a bad thermostat is overheating. If your car’s engine is running hot and your temperature gauge is creeping into the red zone, it could indicate that your thermostat is stuck in a closed position. On the other hand, you may not know that overcooling can also be a sign of a thermostat issue. If your engine isn’t reaching the right temperature, it could be a result of a thermostat being stuck open, which can lead to overcooling and potential engine damage.

Strange Sounds And Temperature Changes

One particular indication of a faulty thermostat is the occurrence of peculiar sounds such as rumbling, boiling, or knocking coming from the radiator. These sounds are all clear signs of an issue in the cooling system, which can lead to larger problems down the line. Another symptom to watch out for is erratic changes in temperature coming from the vents. If the air suddenly goes from hot to cold or vice versa, it may be a sign that the thermostat is not functioning properly.

Coolant Leaking

If you have experienced overheating or noticed liquid dripping from underneath your car, it may be a sign of a coolant leak. However, it is important to note that this may not necessarily be caused by the thermostat itself, as it doesn’t actually contain any coolant. Instead, the issue may be a faulty gasket, preventing coolant from properly flowing through the thermostat housing. It’s important to check for any signs of leaks around the housing, including drops, drip marks, and deposits.

Related: Transmission Overheating: Symptoms, Causes, Solutions

Heater Problems

Heater problems, specifically with the thermostat, can easily go unnoticed until the winter season sets in. If you find that your car’s heater is not producing hot air despite turning up the temperature, it could be due to a faulty thermostat. It’s important to address this issue promptly to avoid driving in the cold with a malfunctioning heater.

So, how do you know if your car thermostat is bad? Look out for signs such as the heater not producing hot air, the engine temperature gauge not working properly, or the check engine light appearing on your dashboard.

How Much Does It Cost To Replace a Car Thermostat?

How-Does-A-Car-Thermostat-Work (2)
How-Does-A-Car-Thermostat-Work (2)

On average, a professional mechanic will charge between $140 and $300 for the service, depending on the make and model of your vehicle. While the thermostat unit itself can cost between $20 and $80, luxury or sports cars may need a pricier replacement. Keep in mind that the labor cost can also add up, ranging between $120 and $220.

Does A Car Thermostat Open and Close While Driving?

While driving, the thermostat is never fully open or fully closed, as it adjusts based on the conditions of the engine. This allows for the proper flow of coolant to keep your engine at the optimal temperature. The water pump does its part by circulating the lower temperature coolant from the radiator into the engine.

Related: How to test a fuel pump

FAQs about How Does A Car Thermostat Work?

What does a thermostat do in a car?

A thermostat works to regulate the flow of coolant between the engine and radiator based on the engine’s temperature. By keeping the engine isolated from the radiator until it reaches a minimum temperature, it ensures that it quickly warms up.

How often should I replace my car thermostat?

Most car manufacturers recommend replacing your thermostat every 100,000 miles or 10 years. While you can wait longer to replace the part, it’s important to pay attention to any signs that may indicate a malfunctioning thermostat such as overheating and strange noises coming from the radiator.

What happens if a car thermostat fails?

As your engine heats up, the thermostat opens to allow coolant to reach the radiator and keep things running smoothly. However, if it begins to fail, the opposite occurs – your engine overheats and can even boil over. Talk about an expensive repair bill!

Related: How to unclog a radiator


So, now you know why cars need thermostats and how they affect different systems in a vehicle. Additionally, we have covered the symptoms of a bad car thermostat and what could happen if it fails.

Keeping an eye out for signs such as overheating or strange noises can help you diagnose any issues early on so that you can avoid expensive repairs. Remember, replacing your car thermostat every 100,000 miles or 10 years is the best way to ensure that it’s functioning properly.

Thank you for reading! We hope this article was helpful in answering the question how does a car thermostat work.

Leave a Comment