Symptoms of a bad or failing thermostat

Five Bad Car Thermostat Symptoms

1- Overheating

Vehicle overheating is one of the major bad car thermostat symptoms that can cause fatal consequences. When the temperature of the engine reaches its normal operating temperature, the thermostat allows the engine to control the heat. 

What happens when a thermostat gets too hot? If the car thermostat is bad, it won’t allow coolant from flowing into the engine, which ultimately causes the car to overheat. So when this happens one of the possible causes is a broken thermostat.

Can you drive a car with a bad thermostat? If you continue to drive a vehicle in this condition, it increases the chances of damaging the engine in the long run. This is why it is important to keep an eye on signs that the vehicle thermostat is not working to avoid any danger during a drive.

2- Heater Malfunction

Take your car to a mechanic, if you find signs of
Take your car to a mechanic, if you find signs of a bad thermostat in your home

Locking the thermostat in an open position is one of the reasons car heater not working. The coolant will constantly flow in the engine regardless of whether it is warm enough to need it. If you turn the car’s heater on and the thermostat is open,  cool air will constantly come out from the HVAC even if you raise the heater temperature. In other words, instead of getting too hot all the time, the engine has a problem with being too cold.

3- Temperature Fluctuations

A faulty thermostat will result in strange temperature changes during engine operation. In this case, the thermostat will not get stuck in one position, but there will still be false readings causing control issues. The engine temperature will change unpredictably and the radiator coolant will not flow normally.

4- Poor Engine Performance

A defective thermostat will affect your car’s engine performance because the car’s engine runs ideally at a certain temperature? between 90°C to 104°C. While overheating of the thermostat can degrade performance, low engine temperature can also impair your car’s engine performance. 

5- Rumbling Noises

Always check the signs and understand how does a b
Always check the signs and understand how does a bad thermostat look like

One of the notable symptoms of a stuck open thermostat is that you start hearing rumbling noises from your vehicle. The noises may come from the vehicle’s engine, radiator, or both. The noise can be heard as a boiling sound, gurgling sound, or knocking sound. Therefore, when you start hearing such noises or experiencing the above bad car thermostat symptoms, you most probably have a bad thermostat engine.

How Does A Thermostat Work?

A thermostat is an integral part of the car’s cooling system. The only way to fully understand it is by spending some time understanding how it works.

The car thermostat is a device that responds to the data it gets about the temperature of the engine. The coolant, once it flows through the engine block, will gain in temperature by heat transfer. By reading this data regarding the temperature of the coolant, the thermostat can find out the temperature of the engine.

Once the coolant is administered through the engine, if it’s cold, the coolant will circulate till it heats up. Thus, an engine is at its optimal temperature. If the coolant is too hot it will enter the radiator in an attempt to bring its temperature lower. A simple mechanism but highly effective.

How Does It Open And Close?

This action is what ultimately controls the flow of coolant in the engine.

A wax cylinder is equipped beside the engine. It’s fixed in the direction of the thermostat. This way, the wax will be in contact with the engine’s coolant and thermostat. The cylinder of wax is also pressed with a rod to its center.

When the temperature of the engine rises, so does the coolant. Since the coolant is in contact with the wax, it is going to increase the temperature of the wax as well.

The increased temperature is going to melt the wax. As the wax melts, the volume of the wax will increase as it expands. The expanded wax is going to push the rod outwards from the center.

This movement from the rod is going to open the valve and the coolant is going to flow to the engine block. When the engine is cold enough, the wax is going to solidify again and the pressure pushing the rod outwards is going to decrease. This, eventually, will cease the flow of coolant into the engine.

Once you start the engine after long periods of inactivity, the coolant will not flow, allowing the engine to heat up and obtain its optimal temperature.

When the engine is operational, the thermostat will never be in a completely closed state. It will be open just enough to maintain optimal engine temperatures.



The vehicle’s engine should be kept in a specific temperature range from $195 F to $220 F. 

The thermostat is the part that is responsible for keeping the engine’s temperature within the optimal range. The thermostat works as a valve to control the flow of the engine’s coolant. 

When the engine’s temperature rises very high, the thermostat opens and allows the coolant to flow to the radiator that reduces its temperature. On the other hand, once the coolant got to the required temperature, the thermostat closes.

A bad thermostat can stuck-open or stuck closed. A stuck-open thermostat can prevent the engine from getting heated enough to function properly. A stuck-closed thermostat is a much more serious case where the engine overheats quickly and might be self-destructed.

Despite the reason for the thermostat problem, it is recommended that you get it fixed as soon as possible. 

Replacing a bad thermostat cost can range from $30 if you fix it on your own to couple hundred dollars if you had it set at a dealership. Therefore, experts recommend that you replace the thermostat on your own if you have the required mechanical skillset. 

Thermostat Location

The thermostat is often located in a plastic or me

The thermostat is often located in a plastic or metal housing near the water pump, connecting to the radiator’s lower hose.

It is most often located on the housing that connects the radiator’s lower hose, but it can be the upper hose in some cars.

Because it is often installed inside a housing, it is in most cases impossible to see with your own eyes without removing it, so to use a repair manual is a good way to find the exact location of your car model.

3. Temperature Changing Erratically

Erratic temperature fluctuations can also occur, causing dramatic spikes and drops in temperature that eventually lead to poor engine performance and reduced gas mileage. In this case, you may see the temperature abnormally low at one point and, shortly after, climb to an abnormally high level. The thermostat itself isn’t stuck in any one position, but it will still produce false readings and cause issues with coolant regulation.

Cabin Temperature Changes

Higher Cabin Temperature / Representation
Higher Cabin Temperature / Representation

Another common symptom of a failing temperature gauge is sudden temperature changes inside the vehicle. If you happen to notice that the temperature inside your cabin suddenly drops and then rise again, this can be a sure shot symptom of a failing thermostat that requires replacement.

GoMechanic Spares: One Stop Destination For Authen
GoMechanic Spares: One Stop Destination For Authentic Spares

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.

What are the bad thermostat symptoms?

As we mentioned before, the thermostat’s job is to switch between the opening and closing cycle.

Bad thermostat scenarios are very straightforward to understand. 

If your vehicle’s engine’s thermostat went bad, then one of the following scenarios happened: 

  • The thermostat stuck open

If the thermostat is open all the time, then the coolant fluid will continuously pass through and go to the radiator. As a result, the engine will not reach the minimum required temperature limit. 

In continuously cold engines, the oil will not lubricate the engine appropriately, which results in too much tear and wear and engine failure eventually.

Besides, if the engine did not reach the required temperature, it will not provide the necessary power for the vehicle to move, especially at higher speeds.

The second scenario is to have a stuck closed thermostat. This scenario is much more dangerous than the first scenario. 

An overheated engine can result in fast complete engine failure. In other words, it will be a brief period for the driver to be self-destructed.

Despite the scenario, a bad thermostat will cause engine failure, and the difference is just the time. Usually, if you used a code scanner to check what is the car’s problem, it will most likely be P0128 (coolant thermostat problem; temperature below/above regulate temperature.)

The question now, how do you, as a driver or a vehicle owner, know what is going on with your vehicle? 

There are common adverse thermostat symptoms, including:

  • The temperature gauge on your vehicle’s dashboard shows a very high temperature (overheating). You might see this within about 15 minutes from firing your engine. This is an indication of a stuck-closed thermostat problem. 
  • Erratic Temperature changes. Sometimes, you see the temperature gauge on your vehicle’s dashboard, showing very high temperatures, followed by very low temperatures. This is an indication of a bad thermostat functioning. 
  • Coolant leak. If you see a puddle of fluid under your thermostat housing, this might be the engine’s coolant leaking. The engine’s coolant is responsible for maintaining the temperature around the engine. If there is no enough fluid in the engine’s coolant pipes, the engine will not function properly. 

A coolant leak can be due to a stuck-closed thermostat. The continuous closure of the thermostat along with the pressure acting on the coolant can result in coolant leaking around the thermostat housing. In more serious scenarios, the coolant might leak from both hoses surrounding the radiator.

Other broader symptoms that you might experience if you have a bad thermostat might be:

  • Temperature gauge reads lower or higher temperature than usual
  • The vehicle’s head does not work
  • Your automatic transmission is not able to shift to higher gears due to the lack of power.
  • A significant drop in the fuel economy.
  • Temperature warning light illuminating on the vehicle’s dashboard
  • A failure in the head gasket
  • And finally, self-destructed engine

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