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Car Thermostat Problems - How to Detect and Fix Them

Educational  ·  April 10, 2023

Car Thermostat Problems - How to Detect and Fix Them

Internal combustion engines generate plenty of heat, yet they don’t overheat thanks to their onboard cooling systems. The thermostat is a critical component in this system, so it’s crucial to understand how they fail, what the symptoms are, and how to fix them.

Car thermostats open and close as needed to control coolant flow. They can only fail when they get stuck in either the open or closed position. You can detect that when the engine overheats or doesn’t heat up enough. You may also notice erratic changes between extreme temperatures. The best fix is to replace the thermostat with a new one.

A bad thermostat can lead to engine damage and expensive repairs. With this guide, it will be easier for you to detect a thermostat problem and quickly resolve the issue, but first, the basics.

What Is a Car Thermostat, And How Does It Work?

Before troubleshooting any car component, you must first understand how it’s supposed to work. So, let’s take a quick look at what a thermostat is and how it’s meant to function.

The thermostat is a removable valve attached to your engine’s cooling system and is critical to prevent overheating in the engine.

A common misconception among drivers is that coolant continuously flows through the cooling system while the engine runs – and that’s not true at all.

What happens is that coolant only flows when the engine gets hot enough and reaches a specific temperature. That’s when the thermostat opens, allowing coolant to flow through the cooling system.

That will continue for as long as necessary to keep the engine operating within optimal temperatures.

Car engines cannot function optimally when too cold. So, when the engine temp drops below optimal operating temperatures, the thermostat will close and halt coolant flow, allowing the engine to maintain the heat it needs to work.

The car thermostat will open and close as often as necessary to maintain the temperature within safe ranges.

What Are the Symptoms of a Bad Thermostat?

As mentioned, a car thermostat functions works in a straightforward manner. It does two things: open and close as needed to prevent overheating and overcooling.

There are only two ways it can fail - getting stuck in the open or closed position. 

For both cases, the best fix is to replace the thermostat with a new one. Fortunately, they don't fail often, so you'll only replace them once, if at all.

The tricky part is that the thermostat is not exactly in your direct line of sight when you check the engine bay. What it means is that you will only know if the thermostat has gone bad if you inspect it up close.

Thankfully, there are several other symptoms you can rely on to detect a bad thermostat.

These are those symptoms:

1. Overheating

If your thermostat gets stuck in the closed position, the coolant won’t flow through the cooling system even when your engine gets too hot. As a result, your engine will quickly overheat.

The red light (engine temperature indicator) on the instrument panel is your first clue to this problem. If you let the problem continue without shutting your engine off, you'll quickly experience steam and loud noises from the engine, so don’t ignore this warning.

2. Overcooling

A car thermostat can also get stuck in the open position, causing the coolant to flow continuously and overcool your engine. In short, the engine will never get a chance to reach its optimal temperature range.

The blue engine temperature light on your instrument panel will be your primary indicator that this problem is happening. Typically, the light shuts off a few minutes after you start your vehicle.

But if it never turns off or the light suddenly comes on after driving for quite a while, your engine is overcooling. Again, the car thermostat is likely to blame.

3. Intermittent Temperatures

As you read above, your car’s thermostat can fail in one of two ways: by being stuck open or stuck closed. However, a faulty thermostat can also alternate between the two.

In other words, intermittent temperature changes are also a symptom of a bad thermostat. For example, your engine might overheat one moment and overcool later.

The engine alternates between two extreme temperatures because the thermostat fails to keep it in the optimal temperature range.

4. Heater Issues

You can also tell if your car thermostat is bad by observing your car heater, especially during cool weather.

A car's heater collects heat from the engine's cooling system. If the thermostat is stuck in the open position causing an overcooling condition, you won't get hot air from the vents.

5. Unusual Sounds

Temperature extremes aren’t the only symptoms of a bad thermostat. You can also detect car problems by listening for unusual sounds like:

These sounds are due to the overheating cooling system causing the metal parts to expand and boiling the trapped coolant. 

How Do You Test a Bad Thermostat?

When you suspect your thermostat has gone bad, you can test it using your kitchen stove.

Firstly, wait for your car to cool down completely before removing the thermostat.

When you remove the thermostat, it should remain closed at room temperature. Then, placing it in a pot of boiling water should cause it to open within 10-15 seconds.

If the thermostat fails to open or close correctly, it’s gone bad. You’ll want to get rid of it and purchase a replacement for your vehicle.

Is It Safe to Drive with a Bad Thermostat?

No, driving with a bad thermostat is unsafe, regardless of the position (open or closed) the thermostat is stuck in.

Firstly, a bad thermostat stuck closed will cause the engine to overheat quickly. Driving under those conditions will cause the engine to damage itself and other components from within, leading to expensive repairs and parts replacements.

On the other hand, a bad thermostat that causes excessive cooling is also bad for the engine. The engine’s components, fluids, and lubricants are all designed to operate at a high temperature.

When the engine is too cold because of a faulty thermostat, the engine's components will experience excessive wear fast. Driving the car might not cause it to overheat, but you're still damaging the engine and its components.

Car thermostats are easy to find and inexpensive. You can browse CarpartAU to find the one you need on the Marketplace or submit a request through the Part Finder today!


By Ray Hasbollah

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