Step by Step: How to Clean a Furnace Flame Sensor

Flame sensors are an integral part of any furnace, but what happens when they aren’t working properly? 

One of the most common furnace problems homeowners notice is their heat starting and turning off again a short while later. You might even notice an error message on your furnace along the lines of “ignition failure.” 

So what does this mean, how do you diagnose it, and how do you safely fix it? Read on to find out. 

Before You Start

WARNING: Before doing anything, shut off the power to your furnace. 

There should be no electricity running to your unit while working on it. If you are not comfortable or unsure about working on your furnace, call a heating and cooling professional - they’ll be happy to help. 

1. Locate the Burner Compartment and Flame Sensor

Next, it’s time to get acquainted with your furnace if you are not already. You will want to remove the panel that covers the burner compartment. The burner compartment is a box that contains the gas line and a set of burners.

By the way, you can follow along step-by-step on our Youtube Channel - click on the images in this post to join our flame sensor cleaning video at each step.

Locate your flame sensor. It looks like a small stick located below your burners inside of the burner compartment. 

You’ll notice above there are two “sticks” in the burner compartment - so how do you know which is the flame sensor? 

If you look directly below the flame sensor, you’ll see one wire running to it. The other stick, with two wires running to it, is the ignitor. 

Your flame sensor will have a single wire running to it. On the right, you can see two wires running to the ignitor.

2. Remove and Inspect the Flame Sensor

You should see a screw holding the flame sensor in place. Using a screwdriver, remove the flame sensor from the unit. You should be able to fully remove the rod of the sensor, along with the wire that is attached to it. 

A dirty flame sensor is typically covered in carbon residue, a soot-like material that is the by-product of combustion. It’s completely normal, but it can impact the flame sensor’s ability to conduct electricity and, thereby, detect a flame. 

3. Clean the Flame Sensor

Now for the most important step - actually cleaning your flame sensor on your furnace. At DallAire Heating and Cooling, we like to use a bronze brush. Some people use a green Scotch Brite pad or a dollar bill.

The most important thing is to not score or scratch the surface of the flame sensor. Using products like sand paper or steel wool could scratch the surface, resulting in a flame sensor that collects carbon deposits more quickly, meaning you’ll be re-cleaning your furnace flame sensor multiple times throughout the year.

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