How to Clean A Dirty Flame Sensor

Furnaces are an important part of our home’s daily environment. During the late fall and winter months, furnaces are often used to provide our families with warm air to push away the cold of the outdoors. However, furnaces are also very complex pieces of machinery, even though sometimes, some repairs seem to be quite simple. When your furnace’s flame sensor becomes dirty, the entire system can stop.

Thankfully, the furnace flame sensor is one of the system’s easiest components to find and clean. Even if the part needs to be replaced for whatever reason, many flame sensors aren’t all that expensive to replace. Though the sensor is an important part, checking to see if you have a dirty flame sensor can often be the first thing that many HVAC technicians say to start troubleshooting with. 

Today’s article will show you what you can do to get clean furnace flame sensor components the next time you find that your furnace isn’t working. In the case that your sensor is clean and you can’t seem to find the issue, you can call Clover Contracting for help no matter the time of day.

What does a furnace flame sensor do?

If you aren’t as well versed with the parts of your furnace, the flame sensor is one of the many safety features built into the appliance. 

Though typically found on gas furnaces, the flame sensor checks to see that the right amount of fuel is used to ignite the burners

When the furnace creates a spark to light the gas or switches on the hot surface ignitor, the flame sensor sends a small current of electricity to the furnace’s circuit board. If that circuit board doesn’t get the proper reading, the system stops flow of gas to the furnace in order to prevent an explosion or overheating. 

Why do you need to clean the sensor?

As mentioned before, the furnace flame sensor is what tells the HVAC circuit board that the appropriate amount of gas was used to create the flames. With a dirty flame sensor on gas furnace units, the sensor can’t effectively pick up how large the flames are or how much gas was used, even if it was the correct amount. As a result, the dirty flame sensor sends the wrong amount of electricity and stopped the furnace from turning on. 

Over time, the dirt and other debris on the flame sensor can cause oxidation or carbon build up, further restricting how the part works and preventing the furnace from turning on. Eventually, the buildup can damage the sensor, needing to be replaced and not letting the furnace turn on until it is.

Causes of a dirty flame sensor on a furnace

There’s only really one main reason why your furnace’s flame sensor is dirty and why you’d need to clean furnace flame sensor components. Usually found in or near the furnace’s burner assembly, the device is usually shaped like a thin bent rod sitting close to the flame stream. Because of its function, the sensor is highly sensitive and variations can often cause the entire furnace to stop working in order to keep the home safe. 

As a result though, any dust or fine debris that can make it past the furnaces filter can stick to the flame sensor. Once the flame stream ignites, the debris burns onto the sensor. With enough time, the buildup of dust and carbon causes the flame sensor to malfunction and send an incorrect reading. The flames shut off and the furnace stops.

In some cases – though it can be rare – the flame sensor might just not be proper for the furnace. Whether it’s not the right size for the furnace or just a bad sensor, the only real solution to this kind of problem is to replace the part with a new one. If the flame sensor is clean, do make sure to check for any other potential causes instead of immediately replacing it, though. While it may be an inexpensive replacement job, no homeowner wants to just waste their money.

How to clean furnace flame sensor components

Continue reading to learn how you can clean a furnace flame sensor in three simple steps:

Shut off the power

To clean furnace flame sensor components, the first step you need to do before you actually start is to turn off the power to your furnace. This can usually be done at the special HVAC breaker that’s found outside your home, but you might also be able to find a switch closer to the unit itself. 

If you see an on-off switch like the ones for your lights, you can use that to turn the furnace off. Otherwise, find the appropriate breaker and shut it off. You will also likely need to turn the gas off if your furnace works with gas.

Find and remove the flame sensor

As mentioned, the flame sensor is typically found close to the furnace’s burner assembly and close to where the flames are ignited. If it isn’t there, you may need to locate it with the manufacturer’s manual or find it yourself. 

Once you have, remove it using a ¼ inch hex headed screw bit. Be careful when removing it, since the part can be sensitive and may have a wire connected to the end of the sensor that you may need to disconnect. 

Clean the sensor and place it back

To actually clean the sensor, use a very light grit sandpaper to lightly rub at the metal rod – and only that part of the sensor. Rubbing at any other part will likely damage the sensor entirely. 

Then, after wiping away any dust, reconnect any wires and screw the sensor back into place. Open the main gas line if you closed it and turn the power back on to check. With any luck you should have a fully functioning furnace again.


Cleaning a dirty flame sensor can be quite easy once you learn where it is and what the right way to do it is. 

However, if you still find some of the dirty flame sensor symptoms afterwards, like your furnace cycling on and off, it’s a good idea to call a professional.

Homeowners and business owners alike can rely on our talented team at Clover Contracting to help you solve your problem. 

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