Home Improvement Skills & Specialties HVAC

How to Clean a Furnace Flame Sensor

Timothy Dale

Updated on 04/03/23

Closeup of cleaning a furnace flame sensor with sandpaper — The Spruce / Sarah Lee

In This Article

How Often to Clean a Flame Sensor
How to Clean a Furnace Flame Sensor
Tips to Keep Flame Sensors Clean Longer

Project Overview

Working Time: 15 - 30 mins
Total Time: 30 - 45 mins

Skill Level: Beginner
Estimated Cost: $5 to $10

An important part of your heating and cooling system is known as the flame sensor. It's a thin, metallic rod that is positioned in front of the flame stream inside the furnace. When the furnace starts up, gas is burned to produce heat, but if the gas is not igniting, the flame sensor will detect that there is no active flame and automatically trigger the furnace to shut down. This is so that the flow of gas is cut off instead of continuing to seep into the home.

However, the flame sensor may also trigger an automatic shutdown due to a buildup of carbon, dust, or dirt on the sensor. This can occur over time or may be the result of being in close proximity to debris-producing items or appliances, like a litter box, pet bed, or clothes dryer. To keep the furnace functioning properly, it's advised to turn off the power to the furnace, shut down the gas to the unit, and clean the sensor.

Typically, the flame sensor should be cleaned at least once a year to extend the life of the sensor and to improve the function of the furnace. Use this guide to learn how to clean a furnace flame sensor and restore heat to your home.

How Often to Clean a Flame Sensor

A dirty flame sensor can lead to problems with the heating and cooling system, so it's important to regularly clean this part of the furnace to ensure the home is properly heating throughout the winter months. The flame sensor should be cleaned about once every year, though it may require more frequent cleaning, depending on the proximity to any airborne debris-producing items or appliances. Additionally, you should consider replacing the flame sensor about once every three to five years to keep the heating and cooling system working efficiently.

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Equipment / Tools

Screwdriver set
Soft bristle brush

Materials

Emery cloth

How to Clean a Furnace Flame Sensor

Locate the Flame Sensor

Make sure to turn off the gas and the power to the furnace to avoid any potential problems while you are working on the heating and cooling system.

Once the gas and power have been turned off to the furnace, locate the flame sensor on or next to the burner assembly. Remove the access panel to find the flame sensor. Depending on the make and model of the furnace, you can either remove the panel by sliding it to detach it from the furnace or by using a screwdriver to loosen any screws that are securing the access panel.

The Spruce / Sarah Lee
Remove the Flame Sensor

After removing the access panel on or next to the burner assembly, you should be able to find the flame sensor. It is a thin metallic rod that is typically secured by a single 1/4-inch hex head screw. Use a screwdriver to loosen this screw and then slide the sensor out of the furnace. In some cases, you may also need to disconnect the wire from the sensor to allow for easier cleaning.

The Spruce / Sarah Lee
Clean the Flame Sensor

With the flame sensor in hand, use an emery cloth or a soft bristle brush and some electrical contact cleaner to gently scrub the sensor. Keep in mind that the goal is to remove any built-up carbon, dirt, or debris, not to try to scrub the flame sensor until it shines like new. Over-cleaning the flame sensor can actually make the problem worse, leading to the replacement of this important part.

The Spruce / Sarah Lee
Reinstall the Flame Sensor

In the same way you removed it, you will now need to reinstall the flame sensor. If you disconnect the wire for easier access to the sensor, then you will need to reconnect the wire, then slide the flame sensor back into the furnace. Secure the flame sensor with the 1/4-inch hex head screw, then replace the access panel.

The Spruce / Sarah Lee
Test the Furnace

Turn on the gas and power to the furnace, making sure to follow the manufacturer's directions to reignite the pilot light if necessary. Keep in mind that the furnace may take a few extra seconds to start up or the fan may immediately turn on and run for a short period.

This is normal, as shutting down the power to the furnace can cause it to reset, at which point the appliance runs through a series of checks before attempting to start up again. After the furnace has completed these checks, make sure the unit starts up and operates properly according to the temperature set on the thermostat to confirm that cleaning the flame sensor resolved the issue.

The Spruce / Sarah Lee

Tips to Keep Flame Sensors Clean Longer

Over time, a flame sensor will become dirty, regardless of where the furnace is located, so you will need to commit to cleaning this part of the furnace about once a year. However, if you find that you need to clean the flame sensor more frequently, then you may need to inspect the area where the furnace is installed to determine if there are any airborne debris-producing items or appliances.

For instance, many homes are designed with the furnace and laundry area in one room. The close proximity to the dryer can increase the frequency with which you need to clean the flame sensor due to the hair, dust, and lint collected by the machine. Similarly, items like dirty brooms, mops, cleaning rags, litter boxes, pet beds, or even nearby renovation construction could lead to a dirty flame sensor. To keep the flame sensor clean longer, consider moving these airborne debris-producing items to another location in the home.