I had to fix the heater today because it was starting but not staying on. I called a friend (heating guy) to see what to do. He told me about a sensor that probably had build up on it and that was the problem. This sensor is a 1/16” diameter solid rod 4” long that is held directly in the natural gas flame. I took steel wool to clean off a very thin film on the rod. Works fine now. What would the build up be made of? I am thinking Carbon because when Methane is burned it gives CO2,H20 and must be leaving Carbon atoms attatched to the metal rod. It seems extrodenary to me that such a thin build up would stop heat transfer.I would like a bucket of this to paint my house!!!