Pulses manufactured after a break date are supposed to be immune from the problem. It was only starting to show it's head when I left and went to work for Motorola. They are/ were one of the most efficient furnaces made. How often do you have it serviced? The big maintenance item is the air flapper material, and on later ones, the non metallic horn on which they mount. The gas flapper is in the elbow assy down below the silencer, below the gas valve. They can fail, but not often