Furnace has frost on air intake?

Sounds like it's cold where you are. The term is hoar frost. Knock the frost off with the furnace turned off. Check the pipe where the furnace exausts. You may need to extend it a bit to keek it from recycling to the intake. If you extend it, insulate it also. If it is caused by snowfall being sucked into the air intake, consider increasing the size of the pipe with a bell reducer. This in effect reduces the velocity of the air. In worst case situations I have used a "T" after the increaser and then two 90's pointing downward. lc