The thermocouple is a safety device on a furnace. It has a metal rod that attaches to the furnace's gas valve. If the thermocouple feels the heat from a pilot light, it keeps the gas line open. If the pilot light goes out, the thermocouple senses a loss of heat and the rod covers the gas valve cutting off the fuel supply. That's why it's important to know how to keep a thermocouple clean and in working order. Luckily, cleaning or replacing a thermocouple is an easy fix that most handy homeowners can take on themselves.

Hot water heater and furnace in basement
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How Do I Clean a Furnace Thermocouple?

A Dirty Thermocouple

If the pilot light isn't working properly, that could mean that soot has built up on the thermocouple. This buildup can lead to the thermocouple not properly detecting heat, or loss thereof. When a faulty thermocouple gives improper readings, it can shut off the gas supply to the furnace even when the pilot light is working.

How to Clean a Furnace Thermocouple

Before cleaning the thermocouple, shut off the furnace's power supply. Don't worry about turning down your thermostat, as that won't shut off the furnace. If you can't find your furnace's power switch, shut off the circuit breaker that supplies power to the furnace.

The thermocouple looks like a small piece of metal tubing or wire located on or next to the pilot light at the front of the furnace. Use a screwdriver to loosen the screws and remove the thermocouple from the pilot assembly for cleaning.

Use a piece of steel wool or the abrasive side of a sponge to gently remove the soot without damaging thermocouple. Avoid using cleaning products, especially those containing flammable chemicals. Use a pencil eraser to clean between the threads of the screw that attaches the thermocouple. When the thermocouple is clean, reattach it to the pilot assembly. Turn on the furnace and light the pilot light according to the manufacturer's instructions.

Knowing When to Replace Your Thermocouple

Thermocouples can last several years, but they don't typically last as long as the furnace itself. If the pilot remains lit, but the main furnace doesn't stay lit, it's time to replace the thermocouple. Replacement thermocouples are in stock at most hardware and home improvement stores. Because they come in different sizes, take your faulty thermocouple to the store with you so you can make a correct match. If you're still unsure if you have the correct thermocouple for your furnace, a salesperson can help.