How to Remedy a Dirty Flame Sensor for Your Furnace

Even with all the preventive maintenance steps taken, it’s still possible for a furnace to suddenly start cycling on and off. Sometimes it will run for more than a few seconds only to switch off, and right when you need it in the middle of the winter season too. Naturally, homeowners may start to feel a surge of panic since investing in a new furnace would require up to thousands of dollars, and it isn’t an option to go without a heating system in the cold. 

Fortunately, the solution to this problem is usually inexpensive and one that you can spot on your own. Before heading out to look at new furnace models, check if your flame sensor needs a simple cleaning and heating repair. Here’s everything you need to know about identifying a dirty flame sensor and how to remedy it to restore effective heating in your home.

The Furnace Flame Sensor

What exactly is the furnace flame sensor? It’s the thin metallic rod situated in front of the pilot flame in your furnace responsible for lighting the burners after the gas valve releases gas into the combustion chamber. The sensor’s main purpose is to confirm that a fire is present when the gas is open. Otherwise, the gas would have nothing to fuel, and it could leak into your home, which is incredibly dangerous. If the flame sensor detects nothing when the gas is on, then it automatically shuts down the entire furnace system as a safety measure.

However, there are instances where a flame is present, and the sensor is unable to detect it. This prevents heat from being produced and can even cause damages to your furnace unit, resulting in even more expensive repairs.

The Problem: A Dirty Flame Sensor

You might be relieved to find that in most cases, the issue is not necessarily due to defective or faulty equipment but rather a dirty and corroded flame sensor. Since it’s constantly immersed in flame, the sensor tends to accumulate carbon deposits, dust, and other particles. Your flame sensor is very sensitive, so the presence of the slightest deposits can cause it to fail. Fortunately, it’s an easy fix for an HVAC technician. 

Identifying a Bad Sensor

There are several signs that you can easily spot to identify a bad or corroded sensor. 

1. Gas Burners

Check if your gas burners light up. If they stay lit, then your sensor is fine. A flame that goes out after a few seconds is one sure sign of a sensor issue. 

2. Presence of Soot

Another clear sign that your sensor isn’t working properly is if it’s covered in soot. Take a closer look to see if the tip of the sensor is clean or covered in residue. If it is, then your furnace issue is likely due to a malfunctioning sensor.

3. Sensor Casing

Damaged sensors can be seen from the casing. If the porcelain is cracked on your sensor casing, then you’ll need a replacement. This is an insulator that prevents the sensor from being grounded on the furnace bracket. 

Cleaning Your Furnace Flame Sensor

Attempting heating and air repair measures for a dirty sensor is not a one-step process. While it isn’t particularly difficult, it can be exacting, which is why a professional HVAC technician is best suited for the job. 

If you decide to clean or replace the sensor yourself, then turn off the furnace before removing the old sensor. Using light grain sandpaper, steel wool, or a wire brush, gently clean the sensor by rubbing the metal rod only and then wipe with a clean cloth. Reconnect the sensor the same way you took it out, and then power the furnace back on. You can expect it to go through a few checks and cycles before running normally again.


Having a home system as big as your furnace shut down in the middle of freezing winter can be a real cause for concern. While it can be attributed to a number of reasons, a corroded sensor is one of the most common and easily repairable problems. By knowing what signs to look for when your furnace goes haywire, you can easily describe to your HVAC technician the possible issues resulting in a quicker and smoother repair. 

At Air 1 LLC, we are a full-service heating and air conditioning company that provides cooling and hydronic heating solutions for your home or commercial establishment. Get in touch with us to get a free estimate from a licensed and insured HVAC contractor today!

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