Symptoms of a faulty thermostat (3 Common Problems)

Coolant has a very important job to complete in your vehicle’s engine. Its main task is to remove the heat from the engine that builds up during the combustion process. By allowing the coolant to absorb the heat from the engine it releases it into the atmosphere as it travels through the radiator of your vehicle.

Thermostats could be considered the gatekeepers of your vehicles cooling system. From engines large to small they all complete the same task. They control the amount of coolant flowing through your vehicles cooling system.

To learn more about the thermostat and its vital role in the cooling system check out our great How a Thermostat Works article.  

So here are the 3 most common symptoms of a faulty thermostat.

1. Heater is blowing cool

This symptom is much more commonly noticed in cool climates. When you turn the heater controls to full heat you feel air that is cool, or cooler than normal. This is due to a thermostat that has failed.

When thermostats fail it is most common for them to fail in the open position. If you have read our excellent article on how a thermostat works you may already know what this may cause.

If a thermostat is always open it allows coolant to flow continuously through the cooling system. This results in the coolant never having time to be warmed up by the engine.

Because of this the coolant never will achieve proper operating temperatures and the air that is flowing through the vents will never warm up properly.

This type of symptom can also cause your temperature gauge to read lower than normal. After all, as we begin to understand the system better. If the temperature of the coolant is low the temperature gauge will be as well.

This failure can be repaired by having your thermostat replaced. There are many factors that affect the price of the final repair of this job.

Thermostats vary in price as well as coolant. Expect the price of repair to range from $200-$300.

2. Temperature gauge reading too high

The temperature gauge reading too high is caused by the thermostat not opening. When this type of failure occurs with your thermostat it will cause no coolant to flow through your cooling system.

Because it is not able to flow through the cooling system it will cause the coolant in your engine to heat up rapidly.

This is a dangerous type of situation. I have seen vehicles that have overheated in this type of situation. When engines are overheated, they can cause serious internal damage to the engine.

This type of symptom can be repaired generally by replacing your thermostat. Like above, a thermostat repair will cost between $200-300$.

3. Coolant leak

This is a very common type of failure that I have seen in the industry over my years as a technician. The thermostat housing is sealed with many different types of gasket materials depending on the engine. 

When this gasket fails it causes coolant to leak out. Generally resulting in a puddle of coolant on your driveway. A coolant smell when driving can also be noticed depending on where the coolant leak is travelling.

This type of repair is generally easy and inexpensive to repair. Expect a repair like this on your vehicle to cost between $100-$300.

How to test a thermostat

If you have a suspected faulty thermostat on your vehicle there is a simple way of testing this relatively simple component. You will need a few items to complete this test, thankfully you will find them in your kitchen cupboards.

Items needed: Pot (not one you are going to use again), stove, thermometer, suspected faulty thermostat

1. Fill a pot with water, ensure there is enough water to fully submerse the thermostat.

2. Place pot onto stove, place thermostat in water as well as thermometer.

3. Turn on burner, raise the temperature of the water.

4. With thermostat monitor temperature, between 195 -200 degrees you should notice the center portion of the thermostat begin to open.

5. Remove thermostat from water using a spoon, being careful not to burn yourself

6. When thermostat cools down, the center portion should close again

If you have done this test and found that the thermostat does not open and close properly, it is faulty. A properly operating thermostat should open and close freely. This ensures the proper amount of coolant flows through the engine at the correct time.

Replacing a thermostat

Consult a proper service manual before you attempt to replace a thermostat. Your engine may have specific instructions on how to replace this component so be careful.

A few simple precautions. Ensure that you relieve the pressure on the cooling system before you start. Make sure that the engine is cool, do not attempt this repair when the engine is warm.

Once you have confirmed that the engine is cool, remove the radiator cap to ensure there is no pressure in the system.

Quick mechanic secret. When doing a repair such as a thermostat it is a good time to ensure that your cooling system is properly serviced.

Overtime coolant breaks down, resulting in reduced engine protection. Every time that you complete a cooling system repair it is a good habit to flush the cooling system.

This ensures that any debris/poor coolant that is in your coolant system can be removed. After this we can install good quality replacement coolant that meets manufacturers specifications.

Can I dump old coolant down the drain?

No, this is something that should never be done. Coolant is a hazardous material and should never be dumped down the drain or onto the ground.

Thankfully there are companies that deal with the recycling of these products so they can be disposed of correctly.

Safety Kleen is a leading company that provides the removal of automotive liquids from automotive repair shops.

For the DIYers, I would suggest contacting your local landfill. They will be able to provide you with information on how to dispose of correctly. I know in my area, they take these liquids for free as they do not want people to dispose of incorrectly.

When a thermostat decides to fail it can cause serious issues depending on where you live in the world. Hopefully this article provides you with some more information to help you understand what might be wrong with your vehicle.

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Staff Writer
Staff Writer
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