The Furnace Flame Sensor: A Guide For Homeowners

Posted on: 9 January 2019

The flame sensor inside of your furnace serves the valuable purpose of monitoring when there is a flame ignited. If the flame sensor goes out, your furnace will consistently cycle on and off like it is trying to come on but can't, which can be incredibly frustrating and blamed on several problems. Take a look at some of the things you should know about the flame sensor, its location, and how it is repaired or replaced. 

Where is the furnace's flame sensor located?

The flame sensor is situated inside of the area in your furnace known as the gas burner assembly. This area will also house a pilot light or hot surface ignitor if your furnace is equipped with these features. To get to this area, you will have to remove the access panel from the front of the furnace. You may not have a good visual of the flame sensor itself, which is a thin metallic rod, but what you should be able to see from the outside of the furnace is the porcelain insulating case that encapsulates the sensor to protect it. 

How do you know if the flame sensor needs to be replaced?

It is usually fairly easy to diagnose a bad flame sensor in your furnace with just a little bit of guidance. There are three primary things that will happen if the flame sensor is not working as it should: 

  • The gas burner will light but will not stay lit for more than a few seconds, if that.
  • The tip of the flame sensor is coated in a hefty layer of soot or looks black.
  • The porcelain insulator around the flame sensor is cracked, chipped, or broken.

Of course, if your flame sensor is dysfunctional, you will see issues with the furnace cycling on and off because the flame will not stay lit as it should without a signal from the sensor. 

Is the flame sensor difficult to replace?

The flame sensor is not all that difficult to replace, but it can be a little challenging to get out because of where it is positioned inside of the furnace. Plus, if you do not get the sensor seated just right in its place, you can still have problems with the component doing its job. Because of this, most homeowners prefer to trust a professional heating repair technician for flame sensor replacement in their furnace system. 


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