Signs My Car’s Thermostat Is Going Bad or Has Failed Completely

Car Thermostat Overview

A car’s thermostat regulates the flow of coolant through the engine and is an essential part in the operation of your vehicle. In most cases, an overheating or no-heat condition in your vehicle is due to a faulty thermostat. And since the T-stat price is only about $8, it makes more sense to replace it than to spend hours diagnosing the problem. If that doesn’t fix it, you’ve only spent about two hours learning how to replace a thermostat.

How to Test a Car Thermostat

Before removing the thermostat from your vehicle, check the coolant flow and temperature.

Remove the radiator cap and idle the engine. If the coolant flows right away, your thermostat is stuck open. If it is not flowing, wait until your engine warms up. The coolant should warm to the right temperature and begin to flow after 10 to 20 minutes. If it does not start to flow, but the temperature gauge on your dashboard rises, your thermostat is stuck closed. If your coolant flows normally, there may be another cause of your overheating engine.

To check the temperature of your coolant, start with a cool radiator and engine. Idle your engine and use a thermometer to read the engine block or cylinder head temperature. Then, check the temperature of the upper radiator hose. Wait five minutes and test again. Complete the test a total of three times. If the temperature does not rise by much, your thermostat is stuck open. If the radiator hose remains about the same temperature, but the dashboard gauge rises dramatically, your thermostat is stuck closed. If the upper radiator hose rises to about the same temperature as the engine block, your thermostat is allowing the coolant to flow properly. You may have a different problem causing cooling issues.

To test the thermostat outside of the car, place it in a pot of water on your kitchen stove. Do not let the thermostat touch the bottom of the pot. Use a thermometer to see the temperature at which the thermostat opens. Note the temperature when it begins to open and when it finished opening completely. Remove it from the pot. Observe as it cools to see that is closes gradually. Compare your notes to your owner’s manual.


How to Test Car Thermostat on Your Own

Figuring out how to check a thermostat on your own is not hard, and it can tell you a lot about the shape your machine is in. Before you start testing, get nose pliers, and some gloves.

Put the gloves on and remove CT. Carefully inspect it and pay attention to its position. If it looks closed, everything will be okay. However, if it’s open, you should get a new one. Now, put a pot on a stove and pour enough water to cover the thermostat. When you submerge it, don’t let it touch the bottom of the pot. Use nose pliers for this. Start heating the water and observe the temperature. When your thermostat begins to open, write down the exact degree. Don’t pull it out of the water, wait until it is fully open, and write down when it happens.

When it is fully spread, pull it out of the pot, and then you can compare notes with the specification on your four-wheeler. If the results deviate from the specifications, you need to replace CT.

Why is it important to check the thermostat for failure symptoms?

Preventing our vehicle’s thermostat from failing requires taking into account some considerations. One of these will be to ensure that the coolant is replaced regularly and carefully so that air does not enter the system as this would lead to overheating and/or faulty sensors.

Another factor to consider in preventive maintenance is visual inspection. Dolz recommends inspecting the thermostat at the same time as the water pump and timing belt. Replacing them in one single operation will ensure that the valves are mounted in the correct position and that necessary fixings are avoided.

Related content: Thermostat: Discover its key elements

Next Step

Schedule Car Thermostat Replacement

The most popular service booked by readers of this article is Car Thermostat Replacement. YourMechanic’s technicians bring the dealership to you by performing this job at your home or office 7-days a week between 7AM-9PM. We currently cover over 2,000 cities and have 100k+ 5-star reviews… LEARN MORE SEE PRICING & SCHEDULING

How to Tell If Car Thermostat Is Bad and Failing

Before you decide to pop the hood of your car and try to fix it, you have to know how to tell if your thermostat is bad and failing. However, if you know that there is something wrong with your four-wheeler, don’t hesitate to get down to business, especially if CT is the failing part. When your CT is bad, it will prevent the engine from operating in ideal conditions, which will later affect its overall performance.

Coolant Leaks

Oftentimes when the thermostat fails, it remains in its closed position. As the engine overheats, coolant will overflow out of the thermostat housing. This means that coolant leaking out of your engine could be a sign that your thermostat has gone bad. It can also be a sign that you have a problem with your radiator hoses, the radiator itself, or the seals and gaskets.

Final Thought

At this point, you must have known the symptoms of a stuck open thermostat, symptoms of a bad thermostat, and how to fix bad thermostat problems.

You might notice just one or two of the outlined symptoms when your thermostat goes bad. So, once you notice any of these symptoms, you want to diagnose and replace a bad thermostat, or better still, contact a professional mechanic for proper inspection and repair.

Things to Consider

There are a few things that can help you if your car thermostat not working properly. 

  • It is important to check the thermostat regularly. Also, a little knowledge about a car can save the owner from big issues. 
  • Understand and check the temperature indicator of your vehicle. 
  • Check coolant frequently and don’t use water as a coolant.

The life of a car radiator thermostat is at least 10 years. However, proper maintenance can extend it more.

Symptoms of a Bad Car Thermostat

A failed thermostat will prevent the engine from operating within its ideal temperature range and affect its performance. For example:

  • A thermostat stuck open will cause a continuous flow of coolant, resulting in a lower operating temperature. Since the oil operates below temperature, the condition accelerates parts' wear, reducing engine efficiency and increasing emissions over time.
  • On the other hand, a thermostat stuck in the closed position will prevent coolant flow and cause the temperature to steadily rise. If you fail to notice and keep your engine running, in a matter of minutes your engine self-destructs. Literally.

Either way, your engine will suffer damage. The difference is just in the amount of time it takes. Still, a failing thermostat is not the only cause for an abnormal engine operating temperature.

Other reasons include low coolant level, a bad water pump, a worn-out or loose water pump belt, cooling system leaks, a clogged radiator, a failed radiator fan and a collapsed radiator hose. Whatever the cause, it's a good idea to start looking into the problem before it's too late.

Part 3 of 4: Verify the status of your thermostat

Materials Needed

  • Needlenose pliers
  • Water

If your vehicle is exhibiting any of these symptoms, there is the likely chance that your car’s thermostat housing may have developed a leak due to the excess pressure at the thermostat. This will need to be verified with the following steps.

Step 1: Remove thermostat. Remove the thermostat f

Step 1: Remove thermostat. Remove the thermostat from the thermostat housing.

Step 2: Heat a pot of water. Bring the thermostat into your kitchen and put a pot of water on the stove.

Heat the water to the temperature stamped on the thermostat (this temperature can range from 180 to 212F depending on the type of thermostat), making sure to monitor the water with a meat thermometer.

Step 3: Place the thermostat in the water. Using needlenose pliers, hold on to the thermostat and place it in the heated water.

When the water reaches the specified temperature, you should see the thermostat open. If it does not open or starts to and stops, this thermostat has failed.

If it is the thermostat itself on your car that has failed, this is a quick and easy repair for most vehicles.

NAPA Racing News