We Buy All Cars, Running or Not!

Common Causes and Signs of a Bad Thermostat

signs of a bad thermostat

A car’s thermostat is responsible for maintaining the minimum operating temperature of the engine. It is a small device found between the engine and the radiator. The thermostat in cars functions by blocking the coolant flow to the radiator just until the engine has warmed up. But when the engine reaches its operating temperature, the thermostat will open and allows the coolant to flow. This is done to make sure that the engine can warm up quickly. Over time, the thermostat can become faulty. Signs of a bad thermostat include fluctuating temperature readings, overheating engine, issues with the car heater, blown radiator hose, cold engine, coolant leaks, and poor engine performance.

⚠️ If It's Broken, Don't Fix It - Get Paid Cash for Your Vehicle ⚠️


When you experience any of the signs of a bad thermostat, you need to have it checked immediately. Thermostats usually fail because of age. Normal wear and tear can happen which  can cause their springs to weaken. It can also become faulty due to overheating, sludge, contaminated fluid, or due to some defects. To correct the problem, your car may need a thermostat replacement which can cost between $40 and $450. .

Signs of a Bad Thermostat: How Do You Know If Your Car Thermostat is Working?

How do you know if your car thermostat is working? What are the signs of a bad thermostat? Before we answer these questions, we need to understand what a thermostat is, how it works, and why it is important.


The thermostat is a simple yet important component of a car. It is a valve that sits between the engine and the radiator. This valve is opened and closed via a spring, piston, or through another thermal-controlled device. It is the one responsible for regulating the amount of coolant circulated through the engine and how much coolant is cooled through the radiator before it is recirculated back into the engine. A car’s thermostat responds to the changes of the temperature, ensuring that the coolant in the engine remains within a specific temperature range.


When you start your car after being parked overnight, the car’s thermostat will be in a closed position. But when the engine reaches its full operating temperature, the thermostat will open up and allow the coolant to flow through the radiator. While the hot coolant in the engine flows into the radiator, the water pump also functions by pushing lower-temperature coolant from the radiator into the engine. When this coolant with lower temperature gets into the thermostat container, the expanding fluid will start to contract and cause the thermostat valve to close.


As the engine operates, the thermostat actually never closes or opens completely, but is somewhat almost in either state in order to regulate the flow of the coolant, which can vary depending on the operating conditions of the engine. This process enables the engine to operate at its best temperature. When this happens, it will help with the lubrication, help prevent the engine from overheating, and improve the engine performance.  It also helps reduce emissions and fuel consumption.


To know whether your car’s thermostat is working properly, you can check it and perform a test. You can do so by following these simple steps.


  • You need to park your car on a level surface.
  • If you have just driven your car, you need to wait for a while since it is important that your engine and radiator have cooled down.
  • Once the engine and radiator are cool, open the hood of your car and locate the thermostat. You will usually find it when you follow the upper radiator hose towards the engine. You may see the end of the hose connected to the thermostat housing. Many cars have their thermostats placed in the said housing. If you can’t find your car’s thermostat or you are not sure where it is located, you can consult your owner’s manual.
  • To test your thermostat, you will have to remove your car’s radiator cap and check the coolant flow. To do this, you need to start your engine and let it idle. Look closely through the radiator filler neck and check if the coolant is flowing. During this time, the coolant should not be flowing since your engine has not reached its normal operating temperature yet. The thermostat should be closed.
  • If the coolant is already flowing at this time, it means that your thermostat is open. It is one of the signs of a bad thermostat that could warrant a replacement.
  • If the coolant is not yet flowing, you need to wait until your engine reaches its normal operating temperature. Once it does, look through your radiator filler neck again to see if it has started to flow. If it is not, it means that the valve is stuck closed and it is also one of the signs of a bad thermostat.


Signs of a Bad Thermostat: How Do I Know If My Car Thermostat Needs to be Replaced?

The car’s thermostat is a vital component, but just like other mechanical components of a car, it can eventually become bad or damaged due to some reasons. Thermostats can become damaged due to the following reasons:


  • Age


Thermostats can become faulty over time. As it ages, its internal components can suffer normal wear and tear since it is constantly exposed to extreme heat and cold. It can become faulty gradually and it usually goes unnoticed. You may just suddenly notice the signs of a bad thermostat when it happens.


  • Contaminated fluid


Over time, the coolant might become contaminated and lose its protective properties. When this happens, sludge may begin to show as the coolant congeals. This sludge can find its way into the thermostat which can cause the flow to become restricted or blocked. This problem can also lead to inaccurate thermostat readings which can result in a delay in the thermostat’s opening and closing. When this happens, it can cause a number of problems.


  • Overheating


The thermostat works by responding to the temperature of the engine. If it detects the heat of the engine, it will use that heat to close or open its valve. If for some reason, the engine overheats, it can result in thermostat failure. Since the thermostat is designed and built to handle the engine’s normal operating temperature, when the engine suffers extreme overheating, it can cause damage to the thermostat’s components.


  • Incorrect installation


If you have just replaced your car’s thermostats and you experience some signs of a bad thermostat, maybe it is caused by an incorrect or improper installation. One mistake that some of the mechanics make is when they are installing the bleeder valve or the jiggle valve. They tend to install the said part upside down, not letting air to bleed from the car’s cooling system.


Another installation mistake they sometimes do is installing the thermostat backwards. When it has been installed backwards, the thermostat part that detects the temperature will be placed toward the radiator and not in the engine. The thermostat will still be able to close and open when this happens, but it will take a while for it to function since the heat will have to travel farther before it makes its way to the thermostat.


These are some of the common reasons why a thermostat goes bad. One way or another, you will be able to prevent this from happening by following the recommended maintenance and services set by your car manufacturer. However, the thermostat tends to become bad at some point and it will have to be replaced. You will know if it is due for a replacement when you experience any of the signs of a bad thermostat. The signs of a bad thermostat include:


  • Engine overheating


Engine overheating is one of the signs of a bad thermostat. This can happen when your faulty thermostat is stuck closed. If it is stuck in a closed position, the coolant will not be able to flow and reach into the radiator. The coolant won’t be cooled down which can cause your engine to overheat.


You may notice your temperature gauge go way up into the red mark. When this happens, you can no longer drive your car with a bad thermostat for more than ten minutes since it may start to overheat again and you should never attempt to drive your car with an overheated engine. It can lead to a lot of problems. If your engine overheats, you need to have it checked immediately since it can also be caused by other issues such as a clogged radiator, faulty water pump, or a bad cooling fan.


  • Fluctuating temperature readings


The thermostat valves are perfectly timed to make sure that the engine will be supplied with coolant at the appropriate temperature. When this timing has been altered due to a bad thermostat, the temperature of the engine will be affected and will fluctuate. If you notice that your temperature gauge has fluctuating temperature readings, then it is one of the signs of a bad thermostat. You may notice your temperature gauge go unusually high and then go unusually low.


  • Issues with the heater


When you use your car heater, the heat will be supplied by a heater core. The coolant flows in the heater core from one side to the other which heats the core. When air passes over this core, the air will be warmed up and then enters into your HVAC system that flows through your vents. When your thermostat fails, the heater core will also be affected. If the thermostat fails the heater core might go cold.


A fluctuating heat from your car’s heater is also one of the signs of a bad thermostat. You may notice that the heat from your vents go from hot to cold then hot again. This can happen when the thermostat fails to do its task properly. It has failed to maintain a steady temperature in the car’s engine.


  • Blown radiator hose


When your engine becomes extremely hot, some of your cooling system’s rubber parts might begin to leak or burst. This can happen because when the temperature builds in the engine block, the pressure will also build up. Although the car’s cooling system is a pressurized system, it can only hold certain pressure.


  • Coolant leaks


When the thermostat is faulty, it can restrict the flow of the coolant. If this happens, the hot coolant in the engine will try to escape due to extreme pressure. When the pressure gets too high and the coolant heats up, it can lead to leaks which can occur in many locations. You will usually notice coolant leaks around the thermostat housing or under the car.


  • Cold engine


One of the signs of a bad thermostat is a cold engine or an engine operating at lower temperature. This can occur when the thermostat is stuck in an open position. This will cause a continuous flow of coolant which leads to a lower operating operation. You need to remember that the thermostat’s task is to let the hot coolant flow through the radiator to be cooled, to keep the engine from overheating.


If the faulty thermostat stays open, the coolant will continue to flow regardless of the engine’s temperature. The engine won’t be able to fully reach its full operating temperature and this can lead to a number of problems.


  • Poor engine performance


As mentioned earlier, when the engine won’t fully reach its full operating temperature, it can lead to a number of issues and it includes poor engine performance. This can happen since the engine is designed and built with specific tolerances and it should only operate within a specific temperature range. This means that the operation and the efficiency of your car will be affected.


If the faulty thermostat caused your engine to run cold, your car may be running too rich. When it does, it can increase your fuel consumption and can eventually cause excessive carbon buildup. It can also cause your spark plugs to become foul and increase your car’s emissions.



If you know the signs of a bad thermostat, you will be alerted that there is a problem. You will be able to address the issue immediately and have it corrected. Your car should be checked and the bad thermostat should be replaced in order to prevent any further damage.



© 2022 Cash Cars Buyer. All Rights Reserved. Terms & Conditions | Privacy Policy | Sitemap