Q. Mr. Hardware, my furnace will start to ignite, goes through some startup procedures but shuts down. After several tries, the furnace will run but I know something is wrong. Can you help? Bob in GPF
A. Bob, it sounds like the flame sensor is carbon-ed up.
The flame sensor is a very simple component. It is located above one of the burners, usually in the upper compartment of the furnace. Once you locate the burners, look for a single wire with a ¼” spade (push on) terminal. It will be on the flame sensor, which is held in place with one ¼” hex sheet metal screw.
Pull off the spade terminal and remove the screw holding it in place. The sensor is part porcelain insulator with a 2 to 4” wire that extends above the burner. Remove it and sand it lightly with 150 grit sandpaper or so, (I used a sanding sponge), to remove all the dark carbon off the wire. Once it is clean, it should work for a month or two until you can order a new one.
It’s such a small item, so easy to replace and yet so important for your furnace to function.