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How to Diagnose a Bad Thermostat in a Car

Mechanic holding a thermostat in hand to inspect it

One of the most troubling issues to have with your car is overheating. The issue can usually be diagnosed as a faulty thermostat. The thermostat is the mechanism that controls the flow of heated air to the radiator. So how do you diagnose a bad thermostat on a car?

Check the upper portion of the radiator hose for warmth. If the top part of the radiator hose is cool to the touch, then air is not moving into the radiator. When the hot air isn’t flowing, the car will quickly overheat, which causes significant damage to the vehicle.

Dealing with cars and their intricate workings is one of those endeavors you should not take on if you are unclear about how to replace the thermostat. However, if you have questions or concerns reaching out to a certified mechanic is never a bad option. So read on and learn all you need to know about diagnosing a lousy thermostat in your car.

How to Diagnose a Bad Thermostat

When your car has a bad thermostat, there are a few things you can look at to let you know where the problem is coming from. Some items will be evident, and others could spring up at the most inopportune times. A good rule is always to have tools and safety equipment stored in your car for emergencies.

Old worn out dirty corroded rusted thermostat from a car isolated against a white background

A Car that Won’t Heat Properly Could be a Sign of a Bad Thermostat.

Something that could go unseen in the warm months is a thermostat that is stuck open. When it is open, the coolant flows freely to the radiator. Unfortunately, this means that the coolant never sufficiently heats up to provide defrost or warmth for the vehicle’s cabin.

A few ways to determine if your car isn’t heating correctly are:

  • Defrost – When you kick on the defrost in your car, it will begin to cycle cold air unto the windshield. Give the unit a minute or two for the heat to catch up. The thermostat is stuck open if the windshield remains fogged or no heat emerges from the vents.
  • Radiator Hose – The radiator hose is another place to check to ensure that your car isn’t heating correctly. The hose will radiate heat when the thermostat works to allow warm coolant into the radiator.
  • Heater – The heater in your car works just like the defrost. No coolant will work to warm the vehicle if the thermostat is stuck open. Turn the heat to the max and full force. Allow a few minutes for the heater to warm up.

The car not heating up properly could make it unusable during the winter and wet days. The open thermostat is just the beginning of problems that could see other issues occur with the heating and cooling systems of the car. Repair thermostats quickly to prevent permanent damage to the vehicle.

Close up of a person turning on the heater inside a car on the HVAC climate control via the knob

Temperature Control is the Most Vital Function of the Thermostat

The temperatures inside an engine reach some extreme levels. When they are unmonitored, they can cause explosive damage from pressure build-up that spews coolant hot enough to burn anyone it touches seriously. If this happens, the culprit could be a stuck thermostat. If not, you could be in for a long work day.

A few other ways your vehicle could be overheating are:

  • Radiator – The radiator is often the first to show wear when the thermostat goes bad. It can easily clog, and the thin layers inside the radiator are delicate to changes in pressure. A stuck radiator could see backed-up pressure release inside that punctures the integrity and leaks coolant.
  • Water Pump – The water pump is another place to check if your heat gauges are out of whack. The water pump can stop working and cycle water. When this happens, you get a breakdown effect that spreads to other parts of the coolant system.
  • Radiator Hose – When looking for faults in the cooling system, the radiator is often one of the best and easiest places to check. The water will be ejected with pressure if it becomes unhooked or bursts.

When the thermostat becomes stuck, the temperature regulation in the cooling system goes haywire. By checking the areas around the thermostat, you get an excellent idea of where the breakdown is occurring and can start fixing the problem.

Close up of a car radiator hose inside the engine bay

How to Replace the Thermostat in Your Car

Sometimes all that can be done is to replace the thermostat and hope that fixes your coolant system problems. While it sounds like a process that could be overwhelming for the novice mechanic, it is a straightforward job that anyone with little information and work can make a success.

Locating the Thermostat is the First Step in the Process

The thermostat is always going to be located around the radiator hose. Often it is in the top position of the hose closest to the engine. Its placement allows it to catch the water as it leaves the engine and transports it through the cooling system. The thermostat’s location is different depending on the vehicle but usually never strays from the main pathway of coolant.

Some steps to take before removing the radiator hose are:

  • Protection – The most important thing is your protection when working. You should allow the car time to cool off before removing the thermostat. Wearing gloves and eye protection is also advised as antifreeze could be toxic to the eyes.
  • Coolant Leaking – When you remove the radiator hose, a bit of antifreeze could leak out onto the garage floor. Placing towels or buckets underneath the car is a good idea as a slippery floor while working on the engine could lead to disaster.
  • Spare Parts – Another good idea is to create an area for storing all the bolts and clamps that will be removed during the process. Keeping an area set aside for these small parts will come in handy as losing the pieces could require more work hours.

Once you have the area prepared for the job, going heavy on the towels and buckets is a must, you should be ready to get the thermostat out and replace it. While this is an easy job being safe is the most crucial part. Ensure the coolant is not hot, or you could be in for a scalding that could permanently disable you.

Car mechanic technician removing the old worn out thermostat from the engine

Removing the Clamp is the Next Step in the Process

The hoses in cars are attached via clamps. These clamps circle the hose and can be removed with a set of pliers. When the hose is removed, the coolant will spill out. This is where the buckets and towels come in. The toxicity of antifreeze can harm animals and will do damage to your eyes. Keep the area clean and dry for the safety of all.

The steps in removing the clamp from the radiator hose are:

  • Find the Clamp – the most critical part is to find where the clamp connects to the car. You should look towards the end of the hose as it needs a bit of metal to form the necessary barrier to keep the coolant from leaking. The clamp will be a long piece of flat metal with a pair of access nodes protruding.
  • Loosen the Clamp – With a set of pliers, grasp the access nodes on the clamp and press them together. When pressure is applied to the circular clamp, it will release tension and allow the hose to be removed. Don’t over-stretch the clamp, as some older models could fail to return to standard size.
  • Remove Radiator Hose – Now that the clamp is loosened, you can remove the radiator hose from its connection. This is an excellent time to check the hose for cracks or wear. The hoses are made from rubber with bits of metal or fibers that protect the hose from high heat and pressure.

The clamp could be stiff or need a bit of elbow grease to get it loose. If there’s any dirt or grime on the hose, you should try to clean it off and check the area underneath. Another big problem with closed thermostats is that they can produce enormous pressure if released. These releases come as failures in the clamps or holes in the hose.

Repairing car radiator

Remove the Thermostat

Once the radiator hose is loose, you should see the thermostat. It is a metal frame with a brass cylinder in the middle. The brass is what usually opens and closes. This brass sticking is the major issue with the part, and sometimes a few whacks with a rubber mallet will get it working again. It is easier and safer to replace, which is the best option.

The steps to remove the thermostat from your car are:

  • Remove Bolts – A set of bolts holds the thermostat in place. They will often have a Phillips-head and could be close to the corners or where they connect to the engine’s metal. Remove the bolts and place them in an area where you will not lose them. There could be more or fewer bolts depending on your make and model.
  • Take out the Thermostat – As soon as all the bolts are out and placed in a safe area, you can move forward with removing the thermostat. It is one solid piece and should come out the same way. If not, there could be broken parts in the pieces of the radiator hose. Clean them out before moving on.

Now that the thermostat is out of the way, we can get in the housing and get out problem fixed. The job isn’t over, just halfway, but the light at the end of the tunnel is near. With a bit of rubber work and some replacing, you will be back on the road before you know it.

Mechanic removing the engine coolant thermostat from the vehicle

Replacing the Thermostat is the Next Step

The thermostat will leave a hole that must be filled with the new part. Inside the area, there could be some repairs that you need to make before replacing the piece and some work to the housing itself.

The following steps in replacing the thermostat are:

  • Rubber Works – A bit of rubber, called a gasket, will surround the thermostat. This thick piece of rubber is used to keep constant pressure inside the coolant system. If the rubber isn’t worked back into place correctly, there will continue to be problems with the heating and cooling system.
  • Replace the Thermostat – Now that your rubber gasket is in place, it is time to get the thermostat back in place. The work you did in replacing the rubber gasket should make it a tight or perfect fit. If there’s space inside the housing, ensure it is supposed to be there. Tighten the unit down and make sure there’s no movement.
  • Replace the Radiator Hose – Now that your shiny new thermostat is in its new secure home, it is time to place the radiator hose back where it goes. You should locate the clamp you removed earlier and move it back to the end of the hose. Make sure the clamp is tight around the base.
  • Top-Off Fluids – Chances are that you might have lost some coolant during your removal and installation. Refill all the coolant reservoirs and check their levels before you start your car. Running the vehicle without coolant is like running it without oil. You are asking for a catastrophic breakdown.

Now that your new thermostat is in, you should crank the engine and check the gauges. As the engine runs, move around the area where you were working and look for signs of leaks or other problems. Finally, test the heat and air inside the car to ensure that you have fixed the problem.

Person topping of the engine coolant mixture into the radiator after servicing the cooling system with bubbles visible at the top


Dealing with the cooling system on your car can be a severe headache. Several things could be the culprit of the problem but often, starting with the thermostat and working your way out is the safest bet.

The work required to remove the radiator is one of the most accessible jobs on an engine. Those who have little time working on cars should do their research before jumping in and if they still have issues contacting a certified mechanic is your best option.

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Car Parts

What Are the Common Signs of a Bad Thermostat?

Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

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