How To Fix A Thermostat Not Reading The Correct Temperature (Calibration Guide)

The temperature in your home has suddenly started to fluctuate and some rooms are too cold and others too warm. It seems to you as if the air conditioner and the heating system are working but that they kick in too late. You now wonder whether the calibration of your thermostat might be the problem, and if so, how you can fix the problem

Cooling and heating systems that operate when the temperature in your home exceeds what you’ve set as a comfortable level are often a consequence of faulty thermostat calibration. Temperature changes show poor thermostat calibration. Luckily, you can usually do the needed changes by yourself.

Most of the content in this article is about thermostat calibration, but I’ll also briefly mention other reasons which can cause a thermostat to read temperature incorrectly. In this article, which is a condensed Calibration Guide, I’ll take you to step by step from the stage where you’ve discovered that there is a calibration fault up to where you’ve fixed the problem.    

Signs that calibration might be the cause of your home’s temperature problem

Usually, there are specific signs indicating that your thermostat might have a calibration problem. Let’s look at the two most common signs.  

1. Fluctuating and uncomfortable temperatures

One of the first signs that there might be a calibration fault is when you experience regular temperature fluctuations in your home. With the temperature fluctuation, you also experience that some rooms are too hot or too cold, regardless of the temperature settings you’ve made.

Your home becomes uncomfortable in the summer if the cooling system doesn’t turn on when the temperature reaches the preset temperature, but only starts running after it reaches, for instance, a 5-degree higher temperature. In winter, you have the same uncomfortable effect if the heating system only starts to work when the temperature is say 5 degrees lower than set.

Simply put, it means that if, for example, your thermostat is set to activate the cooling system when the temperature in your house has reached 72 degrees, an incorrectly calibrated thermostat might activate it only when the temperature reaches 77 degrees.

The temperature in your house will also become uncomfortable if your thermostat activates either the cooling or the heating system too early.

2. High Energy Bills

A correctly calibrated and properly functioning thermostat keeps your home at a comfortable temperature throughout the year without using unnecessary energy. When your energy bill is suddenly increasing without any reason, it might be a sign that your HVAC system is running when it is not needed.

The cause of this might be a faulty calibrated thermostat, especially if you are experiencing that your cooling and heating systems are activated more frequently than usual. If they are activated before the set temperatures are reached, your HVAC system is using more energy than needed.

Check Your Thermostat’s Calibration

When you are suspecting that your thermostat is not calibrated correctly, you can follow the following steps to determine whether you have a calibration problem.

  • Step 1: Get a thermometer, and after you’ve wrapped it in a paper towel, place it somewhere close to your thermostat. If needed, you can tape the thermometer to the wall next to the thermostat. Ensure that the thermometer doesn’t receive direct sunlight or is placed in a draft. The thermometer should be placed in such a way that it can reflect the actual room temperature.
  • Step 2: Verify that your HVAC system and thermostat are receiving power and are working.
  • Step 3:  After about 15 minutes, check the temperature readings on both the thermometer and thermostat. Write down the readings.
  • Step 4: If the two readings are the same or within a difference of only 1 to 3 degrees, your thermostat’s calibration is not the problem of the temperature fluctuations in your home. However, if the difference between the two readings is more than 3 degrees, your thermostat needs calibration.
  • Step 5: It is always good practice to repeat this test procedure once more just to verify that the thermostat is constantly reading the temperature incorrectly.

TIP: Even if you are not experiencing extreme temperature problems in your home, it is good to get into the habit of checking for calibration problems at least once a year. Execute this thermometer test to determine whether your thermostat is still reading the temperature correctly. If the readings on the thermometer and your thermostat differ more than 3 degrees, you should recalibrate your thermostat.

The first step before starting the calibration process – clean the thermostat

1. Mechanical and programmable thermostats

Because your thermostat may lose calibration due to dust inside the thermostat, a good cleaning could solve the problem.

If you have a mechanical or programmable thermostat, put off the power to the thermostat because you have to remove the front cover of the thermostat. The best is to put it off at the circuit breaker.

Remove the front cover of your programmable or mechanical thermostat. Take a soft brush and clean the dust inside the thermostat. Use Q-tips or a soft cloth for cleaning. While dusting it, also look for corrosion on the wiring and contact terminals.

If you notice any corrosion, use an electrical contact cleaner and remove the corrosion. Sometimes you’ll have to remove the wire from the terminal to clean it up. When all the corrosion spots are removed, ensure that the wires are secured again.

When everything is clean and reconnected, replace the front cover and put the circuit breaker on again. You can now repeat the thermometer-thermostat test, and chances are great that the issue has been resolved without calibrating the thermostat. However, when the difference between the two readings is still more than 3 degrees, you’ll have to calibrate the thermostats.    

TIP 1: Before you start to disconnect some wires, take a photo of the existing wiring. This will help you to ensure that all the wires are connected correctly after the cleaning process. I always recommend that you remove the corrosion wire by wire and don’t disconnect all the wires simultaneously.

TIP 2: Don’t use a vacuum cleaner to clean the inside of your thermostat. You may damage the delicate parts of the thermostat.

2.  Smart thermostats

Smart thermostats don’t need internal cleaning. Just use a soft cloth to lightly dust off the screen and the area around the thermostat. When you’re satisfied that all “outside” dust is removed, do the thermometer-thermostat test again. If the difference between the two readings is still more than 3 degrees, you’ll have to calibrate your smart thermostat.

How to calibrate your thermostat

1. Programmable and smart thermostats

Always remember that although I provide you with steps that will, in most instances, apply to your thermostat, every thermostat model has different controls. Thus, use the “logical” steps I’m providing and find the exact procedure for your thermostat in its manual.  

Step 1: Go to the Calibration Mode

Your first step to calibrate your programmable or smart thermostat is to enter the thermostat’s calibration mode. Look in your owner’s manual or online for the specific way your model can be put into calibration mode.

Generally speaking, in some thermostat models, you have to turn off the thermostat and press the up and down buttons to get into calibration mode. On other thermostats, you have to go to the menu section and you’ll find the calibration mode in the “Set review swing value” option.

Step 2: Adjust the thermostat

When you are in calibration mode, go to “Settings” and adjust the temperature by pressing the arrow keys. You increase or decrease the number according to the difference in the thermometer-thermostat readings during the temperature test.

To increase the number, press the “+” sign until the selected temperature is displayed on the screen. To lower the temperature, use the “–“ sign.

After you’ve adjusted the thermostat, use the “Exit” or “Back” buttons to exit the calibration mode.

Step 3: Repeat the thermometer-thermostat test again

The only way to find out whether the calibration is now correct is to do the calibration test again. Set your thermostat to the desired temperature and let it run for a few hours – preferably for a whole day. Then take your thermometer and place it in the same spot where you’ve put it with the first test.

If the difference between the readings on the thermometer and thermostat is now less than 3 degrees, the calibration has been successfully done. If the difference is still more than 3 degrees you have to repeat the calibration process. You’ll have to continue with the process until the thermostat is reading the correct temperature.

2. Mechanical Thermostat

If you’re using a mechanical thermostat, you can also calibrate it if it is giving you incorrect readings according to the thermometer-thermostat test. But you must remember that there are two types of mechanical thermostats. One type is equipped with a calibration screw, and the other type has a mercury switch.

Because each type has a different process to follow to calibrate it, you have to know which type of thermostat you have. You’ll find the detail either in the owner’s manual or online.

A mechanical thermostat with a calibration screw

  • To calibrate this type of thermostat, you have to remove its cover by gently taking it off.
  • Then use a wrench to keep the dial you’ll see straight. Now you can rotate the screw without disturbing the dial.
  • Locate the screw which is, usually, located in the center of a curved piece of thin metal inside the thermostat.
  • Use a screwdriver and turn the screw back and forth until you can feel that the contacts have opened.
  • After you’ve waited a few seconds (with the contacts open) turn the calibration screw again so that the contacts can close. Now you’ve reset the thermostat calibration and the issue should be resolved.

A mechanical thermostat with a mercury switch

Your mercury thermostat has a mercury vial. This vial needs to be leveled to function properly and effectively.

  • Turn off the power to the thermostat at the breaker box.
  • Remove the thermostat cover.
  • Use a level to verify whether the sub-base is leveled correctly on the wall. Use the thermostat’s leveling brackets (or the flat area at the top of the thermostat) to place your level.
  • If it is not level, loosen the mounting screws and move the thermostat till it’s level, and then tighten the mounting screws.
  • Put the cover back and put on the circuit breaker. The calibrating issue should now be resolved, but to verify it, repeat the thermometer-thermostat test after a few hours – preferably after a whole day.

Final resort if you don’t get the calibration issue fixed

If you couldn’t fix your thermostat’s calibration problem, the best is to contact your HVAC technician for help. The technician will be able to determine whether it is a thermostat problem or whether there are other reasons for the uncomfortable temperature in your home.

Other reasons for incorrect thermostat readings

For your convenience and to prevent you from spending time calibrating your thermostat while calibration is not the problem, I’ll briefly mention other possible causes of temperature issues in your home.   

  • The thermostat batteries might be low or bad and this can be rectified by replacing the batteries with fresh batteries into the thermostat.
  • The thermostat is placed in a compromised place. If it is in direct sunlight or near outside windows or doors it will not necessarily read the home’s temperature but only the temperature of that specific spot. To rectify this, you’ll have to call your HVAC technician to reroute the wiring to enable you to put the thermostat in a better spot.
  • Loose or corroded wiring can cause the thermostat to read the temperature incorrectly. Check the HVAC system and the thermostat connections.
  • The thermostat might be old and has to be replaced.


Q1: Is it always a thermostat-related issue if the temperature in my house is fluctuating?

A: Sometimes the problem is not thermostat-related. The problem might be with the HVAC system itself. Poor air circulation, frozen AC coils, and dirty sensors can all cause an HVAC system to run improperly.

Q2: What are the two main reasons why a thermostat loses its calibration?

A2: Dust and dirt build-up over time in the thermostat or a sudden power loss are in most instances the reason that the thermostat loses calibration.


When a thermostat’s calibration is incorrect it can make the temperature in your home very uncomfortable. Fortunately, it is relatively easy to recalibrate the thermostat again and enable it to control the HVAC system effectively. It is, in any case, a good habit to check your thermostat’s calibration at least once a year and rectify it if it is not calibrated correctly anymore.  

Charles John

Experienced HVAC technician with 8 years of experience in the industry. Capable of handling all sorts of heating and cooling equipment as well as proficient in operational management, construction-related techniques such as preventative maintenance, electrical troubleshooting and AutoCAD

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