A faulty furnace flame sensor can cause your gas to continue flowing into the burner assembly and potentially ignite the pilot light. The good news is that the flame sensor can usually be restored to proper working order with a simple cleaning. If damage or component failure are present, however, a flame sensor replacement may be necessary.
Getting to the Furnace Flame Sensor
Before you can access your furnace flame sensor, shut off the power to it. Depending on the make and model of your furnace, this may require shutting off the gas valve as well. Next, locate the access panel to your burner assembly and remove it. The flame sensor is located in the assembly and resembles a metal rod with a porcelain coating at the end. It is typically mounted with a quarter-inch hex screw.
Once you have accessed the flame sensor, carefully remove the carbon buildup on it with emery paper or a light-grit sandpaper. Be careful not to sand the rod too much as this can damage the insulation. Wipe the rod down with a cloth or paper towel once you are done.
After the flame sensor is cleaned, you can remount it to the burner assembly with the 1/4-inch hex screw. Reconnect any wires you removed for easier access and replace the access panel. Finally, reconnect the gas valve and turn on the power to the furnace. Make sure you follow your manufacturer’s instructions to reignite the pilot light if applicable.