Tested both thermostats, fuse, flame sensor, and igniter (which all tested fine) and blew out vent line which was surprisingly clean after 4 years. For a 25 year old unit she looks brand new now. I vacuumed the entire unit out and scraped all the lint out, replaced the belt, tensioner bearing, and both rollers with new. Should get at least another 10 years out of it now!