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How to Clean a Dirty Flame Sensor on a Furnace?

how to clean furnace flame sensor

A flame sensor is one of the most essential components in the furnace. The flame sensor acts as a safety device that shuts off the gas to the burners if flame is not detected. If this flame sensor is dirty, it doesn't sense the flame and shut the gas off. A dirty flame sensor is the most common issue that you encounter with the furnace system. Also, it might be caused your furnace to lock out or continue trying to sense the flame. If your furnace fires up and stops within seconds,, then consider that you have a flame sensing problem. Hence, know how to clean furnace flame sensor and keep your sensor clean to work perfectly.

How to Clean Furnace Flame Sensor?

A furnace flame sensor is a thin metallic rod, located on the burner assembly. Normally, the sensor bents and sits just in front of the pilot flame inside the furnace system. When the thermostat indicates that the furnace to turn on, the gas valve opens up and fires gas into the combustion chamber where the ignitor or pilot lights the burners.

The sensor will be there to confirm that there is fire present when the gas valve is open. If there is no sensor present, the pilot light can go out and dangerous gas might be leaked. So, a flame sensor is a safety measure for your furnace. If this sensor doesn't identify the flame when the gas is open, it shuts down everything to keep you safe. In the latest furnace models, the shutdown process can be done up to three times before lockout.

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Causes of a Dirty Flame Sensor on Furnace

The flame sensor will become dirty just because of carbon build-up. Moreover, this sensor is sensitive to variations in the readings that it takes. Even a thin coating of carbon also causes the sensor to read improperly and then shut down. Although, most furnaces are located in basements and attics so tends to be a lot of dust in the air. Over time, the dust or dirt on the furnace burns up and cause to carbon buildup on it's flame sensor.

How to Clean Flame Sensor?

Following are the required tools to clean the flame sensor on your furnace.

  • 1/4 inch hex driver or wrench. This may vary depending on the type of mounting screw.
  • Emery cloth, steel wool or light grit sandpaper.
  • Dry and clean paper towel.

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Shut off Power to the Furnace

Whenever you start the maintenance, you should turn off the power to your furnace. Shut off the thermostat won't turn off power to the unit. So, you have to shut off the power manually. You can have a toggle switch, i.e., an on/ off switch on or near the furnace. If you don't have this toggle switch, you need to shut off the power to the furnace at the circuit breaker box.

If your furnace doesn't have an electronically controlled gas valve, turn the gas off to the furnace also. It is just for safety, don't worry about gas leaks.

As there are many different models of furnaces, these points may not match with you. Then, you should not proceed without shutdown the furnace. Call an HVAC technician to fix the furnace.

Take Out the Furnace Flame Sensor

Once the furnace didn't get power, you have to locate and remove the flame sensor from the unit. The flame sensor is easily accessible and it can be found mounted with a hex head screw. You have to remove that screw carefully.

After removing the screw, simply slide out the sensor for cleaning. This sensor is a sensitive component so you should take care while removing it. Also, you have a chance that you need to disassemble a wire located at the sensor end to work freely.

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Clean the Flame Sensor

It's time to clean the flame sensor of your furnace. Gently rub the rod with light grit sandpaper or scrubbing brush to remove carbon buildup. Make sure you won't rub anything other than the rod.

Here, you are not doing heavy sanding so just give enough force for removing that carbon buildup. After sanding, wipe away the dust that gets left behind from the sanding with a paper towel.

You should not use any chemicals as they leave residue or damage the flame sensor.

Replace the Sensor

The flame sensor doesn't need to spark like new. Just removing carbon buildup is enough before replacing it. Before remounting the sensor on the burner assembly, you should reconnect the wire if it has any. Use the hex head screw to remount. If you lost that screw, you can buy a new screw from a hardware store. Mount the sensor securely and close the furnace door. Next, shut on power to the unit.

Now, turn the gas and electricity back on, fire up the furnace to check it’s working.

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Wrap Up

After completion of flame sensor cleaning, your furnace needs to run normally by setting a degree high temperature. Now, your unit will kick on, reach the temperature and shut off as normally it would. If you feel difficult to know how to clean a dirty flame sensor on a furnace, just call an HVAC professional to fix it.

About the Author


Haritha is a Content Writer at PartsAPS. She has a great understanding of today's marketing strategies that yield strong results and always happy to create valuable content & share her thoughts.

She was good at writing content for E-Commerce websites related to Restaurant Equipment Parts, Appliance Parts, Food Service Parts, HVAC Parts, etc.

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Leave your comment
2/8/2022 6:25 PM
Thanks for this detailed post! I also read a similar <a href="">article</a> that talked about flame sensors. In your opinion, how often does a flame sensor get damaged because of carbon build-up? Also, with the technology now, is there any other way to gauge flame without constant cleaning of carbon build-up?

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