Don't know about you unit but all the ones I have seen you must pull the furnace away from the outside wall in order to remove it. No access from outside. Straight back from the intake and exhaust ports. I'm very sure someone will have an exception to this but that is how the majority work. It requires a special circuit board tester to check the control board unless you can see some burned evidence of damage.There is a sail switch and gas valves involved also. Best to get help. not really a DIY project. You can pull it out and take it to a dealer so you won't be without your rig for weeks.
