Atwood Furnace

BFL13 wrote:
The limit switch must be working for it to be doing that intermittent burning? It can't be stuck open. Too sensitive maybe?

Changing the main board fixed the OP's intermittent problem for almost a year so it wasn't the ducts etc. Then it would not light at all after the fan came on (limit switch stuck open now?). New sail switch fixed that but back to intermittent action. (if they didn't also change the limit switch then it must still not be stuck open)

Has the replacement board from a year ago gone bad? What was that all about with the duff set of boards?--was that furnaces or fridges, can't remember.

The limit switch might be the most likely culprit, but it is the hardest thing to get at, so IMO it is the last thing to try for a DIY. In my case it was the main board at fault, which is the easiest thing to swap out, but last on the list of things to try according to the pros. Maybe getting at the limit switch is easy for a pro! :)

Is there no warranty on this furnace in a 2018 RV ?

You misunderstood what I meant about stuck OPEN. Usually, when you get a furnace that the fan runs but the Gas Valve and Ignitor fail, it is the Hi Limit switch and it is stuck OPEN. When I have this condition, all I have to do is take a long metal punch, insert it by the Hi Limit switch and then pop the punch with a hammer. THAT resets a defective open Hi Limit and then if the furnace fires up, I know to disassemble and replace the Hi limit switch. Sometimes when disassembling to gain access to the Limit you bump the furnace and it resets. You can also go to the 2 white wires feeding the Limit and test for continuity or 12 volts when the furnace fan comes on, but that takes time. Using my punch method gives you a diag in less than 30 seconds. The OP's problem cannot be a defective sail switch. Possible but highly unlikely. Doug