Furnace backfires from exhaust vent

[Sdsjr4112]

Having issues with furnace back firing blowing out a huge flame from gas build up! Runs 3 times igniter keeps clicking like it's not getting enough gas then boom blows fire a time or 2 then kicks it's out the 3rd so I'm assuming sail switch is working properly? Had dirt daubers built up. Took furnace unit out  cleaned it still not fixed! I feel the igniter is bad or it's a sensor of sorts or possibility a bad regulator! Thoughts starting to get cold and need my furnace?? BTW it's a 2021 keystone avalanche 395BH with a Dometic furnace unit! Thanks

[Kirk W]

You are correct that the sail switch is actuated as that has to happen before the propane valve will open or the spark be triggered. It sounds like you still do not have a good air/fuel mixture. the ignition probe includes the heat sensing but either could fail but you should be able to hear the snapping of the ignition spark if standing near it when someone turns the furnace on. I would take a close look at the burner tube at this point. If you post the make and model of the furnace we can then supply you with a link to download a copy of the service manual. 

[Chalkie]

Had the same happen to us and it turned out it was a mud dauber nest deep inside the exhaust. The RV tech had to pull out the furnace in order to position it such that he could get water in to soak the nest soft enough to remove it all. Since then I have made sure to keep a screen over the vents.

I hope it is not as severe, it cost us a couple hours of RV Tech labor time.