How to Clean a Flame Sensor on a Furnace
One of the more common heating service calls has to do with the flame sensor in your furnace. The flame sensor is a safety device that will shut off the gas to the burners if no flame is sensed. When the sensor gets dirty it will act like it doesn't see a flame and shut the gas off. This will cause your furnace to lock out or continue trying to sense flame. If your furnace fires up and the main flame lights and then within seconds drops out, you are having a flame sensing problem.
How a Flame Sensor Works
A flame sensor is a metal rod placed in the path of the flame. When there is a flame present a AC current is sent down the sensor rod and flows through the flame by ionization to the burner, or ground. When the sensor gets dirty it becomes insulated and is not able to pass enough current through the rod and will not "see" a flame.
How to Clean a Flame Sensor
Cleaning a flame sensor is an easy service call. It usually just requires a 1/4" or 5/16" nut driver and steel wool or wire brush. Find the burner section on your furnace by removing the front panel. The flame sensor will be on the opposite side of the burners than the ignitor. The sensor will be secured to the burner bracket by one or two screws. Remove the screws and carefully take out the sensor. Another way to distinguish the flame sensor is by finding the wire going to it. Often times the flame sensor wire is a white wire with different insulation than the other wires. In the picture below notice the white wire running to the left side of the burners. Take your steel wool or wire brush and polish sensor. Using fine grit sand paper will usually get you by, but in the long term it will fail again. The residue from sand paper when heated will become a thin layer of glass insulation, causing the sensor to not work again.
Causes of a Dirty Flame Sensor
As a service technician I've been on a lot of flame sensor calls and I often get asked "How often should I be cleaning this?". Once a year is usually a pretty good answer but really it depends on a number of factors. The biggest factor is where your furnace is getting fresh air. If you have two PVC pipes running to your furnace then the fresh air from outside is piped directly to your furnace. In this type of setup the flame sensor should almost never get dirty. If your furnace has a metal stack then you are using fresh air from the area around the furnace. In this case if your basement is dusty and dirty around the furnace then there is a lot more potential for having flame sensor problems. The biggest problem in this type of setup is not dust however, it's detergents and cleaners. Often times furnaces are in the laundry room and there's laundry detergents and all kinds of cleaners. If you are having a problem with detergents and cleaners it will be characterized by a white powder coating on the flame sensor.