This is the 3rd year in a row that I've had trouble with the gas furnace at start of heating season. Its an Atwood Excalibur 8500-III series controlled by the Duo-Therm 4 button tstat.
The first year we found the control board was bad. Unit was never used and RV techs say the boards have a life span of about 10 years, Needless to say the boards in my Dometic Frig and water heater both stopped working for the same reason.
Second year we spent a week trying to figure out why the furnace would not start. Finally determined it was a defective t-stat that would not send signal to the board. Found a replacement in Colorado and after install, everything normal.
This year, same problem is back with failing to start. Initially we thought it was the board again....got a replacement and still nothing. Then started looking at the sail switch and finally traced it back to the tstat. Took it off the wall and could not see any visual problems and it still functioned normal for the AC and electric heat controls....again, only problem was with furnace.
After it was reinstalled the unit started working but not for long!! I made contact with the CO vendor and he is sending another unit but I must return the old tstat to him for $150. Apparently these have poor soldier points and only rebuilt units are now available.
Sharing the contact info for Bill Hendrix.
Email - WHendrix@aol.com
He responds quickly but requires payment before shipping. I paid thru Paypal and he is sending it Priority Mail. Hope this helps other AS owners with similar problems.