We Buy All Cars, Running or Not!

Bad Thermostat Symptoms. Here’s what you need to know!

signs of a bad thermostat

Bad thermostat symptoms happen suddenly without warnings. You might start your vehicle’s engine and find that the engine is overheating.

Auto Repairs Are EXPENSIVE


The engine’s thermostat is responsible for keeping the engine within the appropriate temperature for the best performance. If your thermostat goes wrong, you might have severe problems in your engine that can lead to vehicle damage. 


Thermostat failure can happen due to many reasons, including problems with the water pump, issues with the engine’s radiator, and damages in the drive belt.


Despite the reason for thermostat failure, you must get your vehicle’s thermostat fixed as soon as possible to avoid significant damages and very high repair costs.


In this article, we will introduce you to the actual job of the vehicle’s thermostat, how does the thermostat go wrong, what are the symptoms of a bad thermostat? How much does it cost you to replace your vehicle’s thermostat? And a step-by-step on how to change your vehicle’s engine’s thermostat by yourself.


How does the engine’s thermostat work?


Similar to any water-cooled engines, the vehicle’s engine has to work within a specific temperature range.


While each car makes, the model has its manufacturer specifications, all engines should work on temperature between 195 F and 220 F. if the engine’s temperature rises above 220, the driver is overheated, and it can be damaged very quickly. On the other hand, if the engine’s temperature kept below 195 F, the oil will not provide the required lubrication and will cause the engine’s components to rust and fail. Besides, the engine will not provide the necessary power at a lower temperature.


The thermostat helps keep the engine’s temperature in the appropriate range. The thermostat works as a valve that allows the coolant fluid to pass to the radiator at a specific temperature and not to move at other temperatures. 


For example, when you just fire the engine, the engine’s temperature will start rising. At this time, the thermostat valve will not allow the engine’s coolant to pass through to the radiator because the radiator is responsible for lowering the coolant’s temperature. When the engine’s temperature approaches the maximum allowed limit (i.e., 220 F), the thermostat valve will start opening step by step until it is fully opened. 


Once the engine’s temperature starts to approach the lower limit (i.e., 195 F), the thermostat will start doing the opposite job and closes gradually until fully closed. 


The thermostat continues the opening-closing cycle to keep the engine’s coolant at a specific temperature range and, therefore, ensures the best engine’s performance.


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 thermostat stuck close



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


How to test the vehicle’s thermostat?


A bad thermostat is easy to find a problem.


There are two ways to test for a bad thermostat: you can check the thermostat while it is connected to the vehicle, or you can pull it out and do a simple test outside the car. 



  • Test the thermostat when it is connected to the vehicle



Before you perform any of the following tests, you need to make sure that the engine is completely cooled down. It is also recommended that you have a friend to rev the engine and to turn it off if it got very hot. 


You first need to locate the engine’s thermostat. If you open the vehicle’s hood, the thermostat housing is located it the end of the upper hose connecting the radiator to the engine. The actual thermostat is sitting inside the thermostat housing box.


When your thermostat is connected to your vehicle, there are two things you need to investigate: 


    • Check the coolant flow: 
      • Open the radiator back.
      • Start the engine and wait till it idles 
      • When you first start the engine, the coolant should not be flowing through the thermostat to the radiator. Simply because it did not reach the required temperature. If the coolant is flowing, this is an indication of a stuck-open thermostat. You can stop the test here.
      • If the coolant was not flowing at the beginning, and it did not flow when the engine got hot, this is an indication of a stuck-closed thermostat.


    • Check the coolant temperature:
      • For this test, you just need to use your hand to monitor the temperature rise in the radiator hoses.
      • Fist start when the vehicle is completely cooled down.
      • Fire the engine and feel the temperature of the hoses. In the beginning, the temperature should be low, and they should feel hotter gradually. 
      • Make sure not to touch the engine when performing this test.
      • Wait about 10 minutes and repeat the previous steps. If the hoses did not get hotter, this indicates that the thermostat is stuck closed, and the coolant is not passing through to the radiator.



  • Test the thermostat outside of the vehicle



For this test, you need to pull the thermostat out of your vehicle, and you will need a kitchen pot, a pair of needle-nose pliers, and a thermometer.


    • Put the kitchen pot on the stove filled with enough water to cover the thermostat.


    • Place the thermostat in the pot without touching the bottom of it. Use a pair of needle-nose pliers for this. 


    • Start heating the water in the pot. 


    • When you see the thermostat valve starts opening, mark the temperature using a thermometer.


    • Keep the thermostat in the pot while the temperature is rising. 


    • Measure the temperature again when the thermostat is fully opened
    • Compare the two temperature values to the vehicle’s specifications. You can find the vehicle’s specifications in the vehicle’s service manual. 


The measured temperature should not differ significantly from the specified temperature in the vehicle’s manual.


If the temperature is completely off the values given in the manual, then you have a bad thermostat, and you need to replace it.


How much does it cost to replace the engine’s thermostat?


In the world of automobile, maintenance cost is divided into parts and labor costs. 


Labor cost is calculated by the number of hours it takes the mechanic to replace the part, which means that the higher the hourly pay for a mechanic, the higher the labor cost. The labor cost can increase significantly when you get parts replaced at a dealership as compared to local mechanic shops. 


Replacing a thermostat cost is one of the repairs that only depends on the labor cost. 

A thermostat part price can range from $30 to $60. On the other hand, it should take the mechanic between one to two hours to get it fixed. Therefore, the labor cost for replacing a thermostat can range from $90 to $100. 


That been said, it is recommended that if you have some mechanical skill, replace the thermostat on your own to cut the repair cost significantly. However, if you do not have enough mechanical skills, do not try to fix it on your own as you might cause many more problems that might cost you a lot of money. 


A step-by-step on how to replace the engine’s thermostat.


If you decided to save money on the thermostat replacement cost, here are the step by step on how you can replace it on your own:


  • Make sure that the vehicle is fixed in place using the parking brake
  • Ensure that the car is completely cooled down
  • the thermoset housing is located at the end of the upper radiator hose, where it connects to the engine. 
  • Remove the two large bolts from the thermostat housing using a wrench
  • Pull out the faulty thermostat
  • Install the new thermostat in the correct place. Make sure to install it in the right direction following the vehicle’s manual.
  • Close the thermostat housing and make sure to tighten the two large bolts
  • Fire the engine and repeat any of the mentioned tests before to confirm that the new thermostat was installed correctly.




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. 


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