How To Test Cars Thermostat Without the Hassle of Removing It

A faulty thermostat can hamper coolant flow into your engine.

Its job is to make sure that the operating temperature within your car’s engine is optimal.

Thus, a bad thermostat will affect the entire cooling system of your vehicle, leading to your engine overheating.

So let’s discuss an easy way to test a car thermostat without removing it from its housing.

Where is my car thermostat located?

The car thermostat is generally placed under the water pump on the cylinder head of your car’s engine.

With most cars, you will just need to follow the upper radiator hose to the engine block, at the end of which will be the car thermostat housing. Some cars can have the thermostat housed at the bottom radiator hose instead.

If you are having difficulties locating the thermostat in your car, review your service manual to pinpoint the exact location.

How does a thermostat work?

A thermostat, by definition, is a device that maintains the temperature in any system.

You can find them everywhere, from household appliances to your car.

In a car, the thermostat is responsible for maintaining normal operating temperatures within the engine, allowing it to perform at its optimal level.

It is sensitive to minute temperature changes and reacts by allowing circulation of coolant from the radiator hose to the engine block.

The thermostat sensor is placed inside the housing and has spring-operated valves which open and close.

As the engine heats up, the thermostat switches to its open position and the engine coolant rushes in to cool the temperatures down.

On the flip side, when temperatures are normal the thermostat valve will be in a closed position, cutting off the coolant circulation to the engine.

Best Way test a thermostat without removing it

Typically, if your check engine light is on, there is a good chance something is wrong with your car’s cooling system. If that’s the case (or even if it’s not), it’s a good idea to give your car’s thermostat a test by following these steps:

Step 1: Park your car on a level surface and make sure the engine and radiator are cold.

Step 2: Turn on the engine and let it idle. Then remove the radiator cap and look into the radiator filler neck to check the coolant flow.

Step 3: Let your engine reach its operating temperature. The coolant will not start to flow immediately. But as your engine warms up, the thermostat valve should open, and the coolant flow should begin.

Step 4: If you see the coolant flowing after some time (5 minutes), your car’s thermostat is in working order, and everything is fine.

Step 5: If the coolant starts flowing immediately, most likely your thermostat is stuck open.

On the other hand, if the coolant circulation does not start even after 5-10 minutes, your car’s thermostat is stuck closed. In both cases, you will need to get a new thermostat.

How to fix a stuck thermostat?

Sometimes the spring-loaded valves of your car thermostat may fail. If that happens, the valves will remain stuck in their last position.

Both scenarios are potentially dangerous for your engine’s health, and they will require you to get it fixed immediately.

It is possible to fix a stuck thermostat by a qualified mechanic. However, it is much more affordable to replace the whole thing.

More often than not, a stuck valve might also mean damage to the sensor, so it is prudent to replace it with a new one.

How bad is a stuck thermostat?

A stuck thermostat is bad, no matter if it is stuck in an open or close position.

However, the severity of your vehicle’s engine will vary. A stuck open thermostat, which is quite common, means more coolant flows into the engine, not allowing it to reach normal operating temperature.

This is not catastrophic if treated quickly. But in extreme cases, it may cause the fuel to condense inside the cylinders leading to a significant reduction in power and mileage.

On the flip side, a stuck closed thermostat means no coolant circulates the engine, thus overheating it quite quickly.

If not taken care of quickly, this can be very bad for your engine leading it to heavily malfunction.

Thankfully, before this happens, your car should give you ample warning in the form of a check engine light and an overheated temperature gauge flashing amber or red.

How to replace a car thermostat?

To replace a bad thermostat, you will need to take it out of its housing first.

And to do that, you will also need to totally unscrew the radiator hose it is connected to.

It’s not a simple procedure, that’s why we recommend you get it done by a mechanic. But, if you insist on doing it yourself, here are a few steps to follow:

Step 1: Park your car on level ground, and make sure the engine is cold before unscrewing anything.

Step 2: Depending on the placement of your thermostat, unclamp and detach the top or the bottom radiator hose using a screwdriver. Keep a drip tray handy under the hood for the excess coolant in the hose.

Step 3: Use a wrench to unscrew the thermostat housing and lift the bad thermostat out. Also, make sure to remove any old gaskets inside the housing.

Step 4: Use the manual to install the new thermostat, spring-side down, and screw the housing back on. Also, carefully reattach the hose to the radiator tank and refill the lost coolant.

Step 5: Switch on your car and let the engine reach its right temperature.

After that, keep monitoring the temperature gauge as you feather the throttle pedal down a good 10-15 times. If the thermostat is installed correctly, the engine should maintain its normal temperature.

Step 6: Take it out on a test drive as you keep monitoring the temperature gauge needle.

What are the other symptoms of a bad thermostat?

Here are a few common symptoms of a bad thermostat.

If you encounter these symptoms while driving, get your car checked immediately. It can be a sign of a deeper problem with the engine or the radiator.

Temperature Gauge

This is the most reliable and common way of sporting an issue with a car’s thermostat.

In normal conditions, when you switch on your car, the needle on your temperature gauge will slowly gain until it settles in the middle.

If the thermostat is stuck open, this process will slow down significantly, and the needle may not even reach the midpoint.

On the other hand, if the thermostat is stuck closed, the engine heats up rapidly, and the temperature gauge needle will shoot beyond the midpoint.

If this is the case, it is always a good idea to pop the hood and check the temperature of the top radiator hose.

If that is as hot as the engine block, then there is a different reason for your car heating up and not a broken thermostat.

Leaking Coolant

This is another symptom of a broken or a thermostat stuck in a closed position.

The coolant will pool inside the housing, as the valve is not letting it flow into the cylinders, causing a leak.

However, leaking coolant can also mean a ruptured radiator hose, loose radiator cap, or a blown head gasket.

It is always good to get an expert opinion before jumping to any conclusion in this case.

Heating Systems

If your car’s thermostat is damaged, it will affect the heating and cooling systems.

In the case of a cold engine, if the thermostat is stuck open, then your heater will also fail to work.

But the good news is that a broken thermostat has little to no effect on the car’s air condition as it is not connected to it at all.

Sights and Sounds

The final symptoms might just be the most obvious ones.

If you can smell the antifreeze inside your car or see smoke coming out of your hood, there is a good chance that your car’s thermostat is busted.


A car’s thermostat is an integral part of maintaining its efficiency, therefore you should not take it lightly.

Hopefully, this article comes in handy, and you can test the car thermostat without removing it.

Remember your car likes to operate at a specific temperature, and it is designed to indicate to you when it is not.

So, no need to panic, it is quite a common condition and can be fixed easily.


Dean Alvarez, TireForge Head Author

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