How can I tell if my thermostat is correct?

Author: Walker Turner MD  |  Last update: Monday, February 23, 2026

One way to check for thermostat calibration issues is to place a room thermometer a few inches from the thermostat and wait 15 minutes. The readings of both shouldn't be more than 1℉ different. If this is the case, then it's time to correct the issue.

How to check if thermostat temperature is accurate?

If both your thermometer and your thermostat display the same temperature reading, the thermostat's accuracy and efficiency are good. A difference of 3 degrees or less is generally nothing to worry about. If the readings are different, there is likely an issue affecting your thermostat's function.

How can I test if my thermostat is bad?

Pull the thermostat off the wall and jumper the red and green wire. If it turns on, thermostat is bad. If it doesn't, something else is going on.

Can your thermostat be wrong?

A common oversight for a thermostat showing the wrong temperature is the state of its batteries. Dead or low-power batteries can lead to inaccurate readings. Simply replacing the batteries might restore your thermostat's accuracy and functionality.

How do I recalibrate my thermostat?

Use the manual to find the screw for calibration. Turn the screw to match the temperature of the thermostat to the room temperature. Turn the HVAC system and the power back on, and the power. Turn on the thermostat and wait a few minutes for it to adjust to the temperature.

How to Tell if Your Home Thermostat is Bad - Bypass it and Find Out

How do you know if your thermostat needs to be reset?

Common Thermostat Issues that Require a Reset
  1. Misreading Room Temperature: If your thermostat reads the wrong temperature, it won't kick the HVAC system on at the right times. ...
  2. Failure to Turn On HVAC System: A thermostat reset reboots its system and attempts to restart normal functioning.

How do you calibrate a temperature?

Fill a large container with crushed ice, and then add clean cold tap water until container is full. Stir. Place the thermometer stem/probe into the ice water. Make sure the thermometer is not touching bottom or sides of the glass Hold the thermometer by its top for 30 seconds or until the reading stays steady.

How do I know if my thermostat is right?

One way to check for thermostat calibration issues is to place a room thermometer a few inches from the thermostat and wait 15 minutes. The readings of both shouldn't be more than 1℉ different. If this is the case, then it's time to correct the issue.

Why does my thermostat setting not match my home's temperature?

Dirty or worn flame sensors, blower motor problems, clogs in the condensate lines, and electrical issues are some of the many problems that can leave your HVAC system failing to deliver the temperatures you've programmed in your thermostat.

Why is my AC thermostat set at 74 but reads 78?

If the temperature is set to 74°F but the thermometer shows 78°F, you will have to recalibrate the thermostat to account for this 4-degree deviation. Even with older AC units or furnaces, you might have to recalibrate your thermostat from time to time.

How do I manually test my thermostat?

Standard thermostats: Enlist a friend or family member to assist, Position one person by the thermostat and one person by the furnace. Slowly turn the thermostat from off to heat and increase the temperature. Both the thermostat and furnace should make a sound.

How do you know if you need a new thermostat?

If your thermostat has fresh batteries but suddenly lacks power, flashes, or isn't responsive to adjustments, it may need replacement. If resetting the thermostat doesn't resolve the problem, you may have wiring problems or faulty components causing your thermostat to not turn on.

Is there a code for a bad thermostat?

P0128 is an extension of OBD-II code P0 which indicates a problem with your car's coolant thermostat.

Can you test if a thermostat is bad?

When you set the thermostat above or below the current room temperature, you should hear a click, and the AC or furnace should kick on almost immediately. If you make changes at the thermostat and nothing happens, that's the first sign you have a bad thermostat.

Can I replace my thermostat myself?

In most cases, installing a new thermostat is a relatively simple do-it-yourself project that can result in improved comfort and convenience and one that can help you save energy.

Do thermostats lose accuracy?

Thermostats can lose calibration over time from being accidentally bumped or from dirt accumulating. An improperly calibrated thermostat can cause discomfort because it's not heating your home accurately.

How do I make sure my thermostat is accurate?

Checking the calibration of your thermostat is easy and should be done once a year, generally in the fall. To do so, locate your home's thermostat and tape an accurate thermometer on the wall next to it. Wait 15 minutes and check the temperature reading on both devices.

Why is my house hotter than the thermostat setting?

Another problem that can cause your house to be hotter than what you've set the thermostat to is if your home's HVAC system isn't sized properly. An HVAC system that's too large for your house will cool your home unevenly. It will only cool some rooms off and then shut off versus cooling down the entire house.

Why does my thermostat say 70 but feels colder?

If you're cold even though your thermostat indicates you shouldn't be, your furnace likely isn't the problem. In most cases, it's working just fine. It's your home that needs attention. Lack of insulation, severe air leakage, and/or unbalanced ductwork are frequently to blame.

How can you tell if your home thermostat is not working properly?

7 Ways to Tell Your Thermostat Is Broken
  1. The Thermostat Has No Power. ...
  2. The AC or Furnace Won't Turn On. ...
  3. The Heater or AC Won't Turn Off. ...
  4. The Thermostat Reading Doesn't Match the Room Temperature. ...
  5. The Thermostat Doesn't Respond. ...
  6. The HVAC System Is Short Cycling. ...
  7. The Thermostat Forgets Programmed Settings.

How do I know if my thermostat needs calibration?

Inconsistent home temperatures and high energy bills are telltale signs your thermostat may need calibration. Thermostat calibration, as well as boiler maintenance, are essential components of HVAC systems. Properly calibrating your thermostat ensures more precise temperature control and energy efficiency.

How do you fix an inaccurate thermostat?

If your thermostat is not reading correctly, it might be due to a dirty or faulty sensor. A quick fix could be to gently clean the sensor with canned air to remove any dust buildup. However, if problems persist, replacing the thermostat could be the next step.

How do you check temperature calibration?

This process involves comparing the readings from the device under test (DUT) with a known reference standard and making necessary adjustments. Accurate temperature measurement is crucial for many industrial processes, from manufacturing and food processing to pharmaceuticals and HVAC systems.

How do I know if my thermometer needs to be calibrated?

They must also be calibrated after an extreme temperature change (e.g., after measuring a hot food first and then a frozen food, or after a thermometer is dropped). All calibrated thermometers should be accurate within 2 degrees (plus or minus 2 degrees).

Previous article
How to plan patio drainage?
Next article
Are all washers high efficiency now?