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How to Inspect & Replace the Thermocouple in Your Furnace

How to Inspect & Replace the Thermocouple in Your Furnace

Conducting regular maintenance on your furnace keeps you and your family warm throughout the winter months. If you are having issues with the thermocouple in your furnace, you can call a licensed HVAC technician to inspect or replace the component. In addition, we have created a list of tips to help you resolve issues with the thermocouple in your heating system. Keep reading to learn more about the thermocouple and other components in your furnace.

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An Introduction to Thermocouples

A thermocouple is a small electrical safety component in a gas furnace. The thermocouple opens the gas valve that feeds into the pilot light. When temperatures are high, the valve opens and when there is no heat, the valve closes. This prevents gas from flowing into the appliance when there is no flame to light it. Thermocouples and flame sensors perform similar tasks but are not interchangeable.


  • Used in older gas furnaces that have standing pilot lights
    Detect whether a pilot light is working correctly

Flame Sensors

  • Used in an electric furnace
  • Sense when the burners are lit and ignites the gas emitted

How to Inspect a Thermocouple

If the thermocouple in a furnace is not working properly, the pilot light will not receive gas to heat the flame. This means you won’t have heat in your home. By inspecting and testing your thermocouple as part of regular furnace maintenance, you can ensure the heat always stays on. You can inspect and test your thermocouple by following these steps. Stay safe and ensure the furnace is off and cool before inspecting.

1. Find the Thermocouple

The thermocouple is found in the flame of the pilot light and is composed of a copper tube, bracket, and wires that connect to the gas valve. Check the manual for your furnace since some thermocouples look different. During your inspection, you are going to look for corrosion on the tube including holes, cracks, and discoloration. In addition, you will need to look for bare wires and missing insulation in your heating system. Inspect the interior of your furnace for damage to connectors and other components. Any obvious damage to the thermocouple will result in abnormal test results.

Tips & Insights: Why Does My Furnace Keep Running Throughout the Day?

2. Conduct An Open Circuit Test

If the thermocouple in your furnace passes the inspection, it’s time to conduct an open circuit test. Tools you will need for the test include a wrench, multi-meter, and either a candle or lighter.

  1. Stay safe and turn off the gas.
  2. Remove the thermocouple by unscrewing the copper lead and connection nut with the wrench. Then unscrew the bracket nuts.
  3. Make sure the multi-meter is set to ohms, not volts. Take the two leads from the meter and touch them together. The meter should read zero.
  4. Turn on the lighter or light your candle. Leave the tip of the thermocouple in the flame until it’s hot.
  5. Connect the multi-meter and the thermocouple by placing one lead on the thermocouple, the other at the end in the pilot light.

Your thermocouple should provide a reading between 25 and 30 millimeters. Anything less than 25 should be replaced.

Furnace Tune-Up Services

How to Replace a Thermocouple in 5 Steps

Deactivate the power and gas to the furnace prior to performing a thermocouple replacement. Tools you will need to replace the thermocouple are open-end wrenches and a screwdriver or nut driver.

  1. Open the maintenance cover. If the furnace has just run, wait for at least a half-hour to allow the thermocouple to cool off before removing it.
  2. Remove the old thermocouple using the open-end wrench. Unscrew the nut connecting the thermocouple to the gas control valve. Loosen the nut that fastens the thermocouple to the standing pilot burner in the furnace. Once the nuts have been removed, you’ll be able to take out the thermocouple.
  3. Shape the lead on the new thermocouple to resemble the old one.
  4. Connect the new thermocouple to the pilot burner and tighten the nuts snuggly.
  5. Place the end fitting of the thermocouple into the gas control valve. Tighten by hand until you cannot tighten anymore. You can finish tightening by using the open-end wrench to twist it a quarter-turn more.

Now you’re ready to turn the gas and power back on.


Furnace Repair and Installation Services

If your furnace is not working correctly, give our team a call by phone at 1-800-642-4419. Our team provides comprehensive HVAC repair services in Trenton, NJ and are available to help you install a new thermocouple in your furnace. Our professionals offer reliable furnace repair and furnace installation services and are committed to keeping your house warm throughout the winter season.

We also offer other types of HVAC services such as boiler replacement, heat pump maintenance, and boiler repair. We have nearly 30 years of experience in the plumbing and HVAC industry. Schedule an appointment online for fast and reliable services. Our team will restore the circulation of warm air in your living space.


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