The furnaces in our rigs are kind of temperamental.. at least ours is. Every year or so, ours won't ignite.. I pull the furnace out and the problem is soot. This same thing happened to me a few weeks ago when I started to get the rig ready to head to Texas for the winter. I got my tools together to pull the furnace out and got to thinking, all I am going to do is blow the soot out of it, so what would happen if I took my air compressor, set it on 100 psi and blew air in through the exhaust port on the outside of the trailer.. I gave it a couple of shots of air, went in and turned up the thermostat and it ignite and has been working fine ever since. You might want to try this before you pull yours because it won't ignite.