High efficency furnaces with LP gas conversion frequently accumulate soot in the heat exchangers, especially the secondary heat exchanger. I think there is a flexible snake for cleaning them; you might check with the dealer or distributor.
They are also prone to burn holes in the primary heat exchanger. I know they are stainless steel but LP burns so much hotter than natrual gas and I have personally seen this problem. To find it, you can rub your hand lightly along the bottom tube feeling for holes or rough spots. If there is a rough spot an extension mirror and flashlight will give you a better look.