How To Inspect and Replace a Furnace Thermocouple: A Complete Guide

Multiple parts within a gas furnace keep you and your family home warm—one of the most important being the thermocouple. This component is essential to gas furnace safety and should be regularly maintained and inspected to keep it in prime condition. In this guide, we’ll cover how to check a thermocouple and how to replace it if needed. 

What Is a Thermocouple and How Does It Work?

Before we tackle the inspection and replacement process, you first need to know what a thermocouple is and how it works. A thermocouple is a small electronic safety component within a gas furnace. On one side it opens and closes the gas valve that feeds the pilot light. On the other end, it reads the temperature of the flame through a metal rod. 

Thermocouples are found in older gas furnaces with standing pilot lights, with the main job of monitoring whether a pilot light is working properly.  

How Does a Thermocouple Make Your Furnace Safer?

A thermocouple keeps your gas furnace safe by monitoring its pilot light and gas supply. Although gas is a great source of energy and is widely used in older homes, it can pose hazards and health risks.

Prevents Gas Buildup

If your pilot light goes out, your thermocouple will close the gas valve and shut off the gas supply. However, a defective thermocouple may not close the gas valve, leading to gas buildup within the furnace. If this happens and gets too close to a burner or flame, then it can cause a small explosion in your furnace.    

Prevents Gas Leaks

Similarly, instead of gas filling and imploding your furnace, it could leak into your home’s air. Gas leaks are dangerous and pose health hazards. A thermocouple prevents a gas leak in your home by shutting off the supply when the pilot light doesn’t ignite.

What Can Go Wrong With Your Furnace’s Thermocouple?

There are a couple of things that can go wrong with a gas furnace thermocouple, which is why regular maintenance is key. One of the most common issues is a dirty thermocouple, which can cause inaccurate readings or prevent a sufficient gas supply to the pilot light. Loose wires or the need for recalibration can also cause trouble.

How To Inspect a Thermocouple in 3 Steps

Now that we’ve covered the importance of a thermocouple and how it keeps your furnace running and your home safe, let’s go over how to inspect it. 

1. Find the Thermocouple

You can find the thermocouple near or within the pilot light of your gas furnace. It’s often made with copper tubing, making it hard to miss. It also consists of wires, a bracket, and other small components that connect it to the gas valve. 

Not every furnace is the same—if you’re having trouble finding your thermocouple, check your furnace’s manual for further directions. 

2. Conduct a Visual Inspection

Once you’ve located the thermocouple, take a moment to inspect it carefully. Here’s what to check for:

• The Copper Tube: Look for contamination like discoloration, cracks, or tiny pinholes.
• The Wiring: Check for wear or corrosion, such as missing insulation or exposed wire.
• The Connectors: Inspect for any physical damage—a damaged connector can throw off your test results.

If everything looks okay and you don’t spot any issues, you’re ready to move on to the testing phase. If not, and your thermocouple is experiencing any of the signs above, call a trusted HVAC company like Total Comfort for assistance. A professional can further inspect your gas furnace thermocouple and provide solutions or furnace repair services.

3. Perform an Open Circuit Test

It’s important to note that if you’re inexperienced working with your gas furnace or not comfortable with conducting the following test, it’s best to call a professional. To perform an open circuit test, you will need a multi-meter, a wrench, and a flame source such as a candle or lighter.

1. First, turn off the gas supply. 
2. Unscrew and remove the thermocouple, using the wrench to release the connection screws.
3. Test the multi-meter by setting it to ohms and touching its two leads together for a reading of zero.
4. Use the lighter or flame source to heat the tip of the thermocouple.
5. Connect the multi-meter to the thermocouple by attaching one lead to each end.

The multi-meter should give a reading of 25 and 30 millimeters. If your thermocouple is producing a reading lower than 25 millimeters, it will need to be replaced. 

How To Replace a Thermocouple

Replacing a thermocouple is something you can do at home, only requiring an open-end wrench and screwdriver or nut driver. 

1. Safety First: Before starting, turn off the power and gas supply to the furnace.
2. Access the Thermocouple: Open the furnace’s maintenance cover. If the furnace has recently been running, wait at least 30 minutes for the thermocouple to cool before proceeding.
3. Remove the Old Thermocouple:
   • Use an open-end wrench to unscrew the nut connecting the thermocouple to the gas control valve.
   • Loosen the nut securing the thermocouple to the pilot burner.
   • Once the nuts are removed, carefully take out the old thermocouple.
4. Prepare the New Thermocouple: Shape the lead of the new thermocouple to match the old one for a proper fit.
5. Install the New Thermocouple:
   • Attach the new thermocouple to the pilot burner and securely tighten the nuts.
   • Insert the end fitting of the thermocouple into the gas control valve. Tighten it by hand until snug, then use an open-end wrench to give it an additional quarter turn.
6. Restore Power and Gas: Once everything is securely connected, turn the gas and power back on to complete the process.

Working with gas can be tricky, and not every homeowner is comfortable with replacing their thermocouple. Rest assured there are professionals near you who can help. Call Total Comfort and schedule a service today.

Call Total Comfort for Thermocouple Replacement & Heating Services

In addition to thermocouple replacement, we offer reliable heating and cooling services throughout the Minneapolis and St. Paul areas. Since 1956 we’ve been trusted in the community, employing NATE-certified technicians and securing 100% customer satisfaction in every service. 

Call Total Comfort for a thorough inspection of your heating system, and we’ll be happy to provide a no-obligation estimate for HVAC replacement or installations.