Effective Ways of Cleaning a Furnace Flame Sensor


If you have been keen enough, you may have realized that at times your furnace starts up and then shuts down as it gets going. It may do this a number of times and then shut down completely. The main reason behind this is a dirty flame sensor. Most of the service work done on furnaces is rather complicated, however, cleaning your flame sensor is much simpler and you can do it on your own.

What a Flame Sensor Does

A flame sensor is a device located at the burner assembly. It is a thin, bent and metallic rod that sits in front of the flame stream in your furnace. The work of the sensor is to confirm to the furnace system that whenever the gas valve is open, a fire is present. If the unit keeps on emitting gas whereas no flame is available to ignite it, an accumulation of unburned gas results. When the furnace starts up and the burners are ignited, the flame sensor will then have a limited time to detect the flame. This short window in detection may cause the sensor not to detect any flame at all and therefore it may shut down the entire unit.

Most units will have an allowable number of shutdowns after which they go into a safety lock out which may last about an hour before trying again. This can ultimately cause wear and tear on the other parts of the unit hence reducing its efficiency.

Causes of a Dirty Flame Sensor and How to Clean It

Carbon buildup within the furnace can make the flame sensor dirty. Due to its low tolerance for variations in the reading it takes, a slight buildup of carbon can cause it to misread thereby shutting down. The location of many units in the attics, basements and laundry areas which are prone to dust makes the particles stick on the flame sensor and later burn causing a carbon buildup.

To repair a dirty flame sensor, you need a clean and dry paper towel, a piece of light grit sandpaper, emery cloth, or steel wool and a wrench.


First of all, you have to shut off the power leading to the furnace and then remove the flame sensor. With a wrench or a quarter inch hex head screw driver, you can unmount the flame sensor and clean it. The grit sandpaper is used to gently rub the metal rod (flame sensor) and thereafter use the clean paper towel to remove any dust or carbon left behind by the sanding. After that, replace the sensor and check the results. In the event the flame sensor is broken, you may have to replace it.