Please HELP - furnace issue

[BigJohn1]

Hello. Really hoping for some help here. I have a 2019 24 foot keystone bullet ultralight. Last week, furnace stopped igniting after I replaced propane tanks. Initially thought it was air in the lines. Bled the lines, did the normal turn on and off, no help. Replaced the tanks, replaced connector hoses The furnace would turn on for 30 seconds, but ignition would not even try to light. Unfortunately, I don’t have an exterior panel to service my furnace. After hours of research, I settled that the issue must be in a cell switch. Followed step-by-step instructions to disconnect and remove the furnace through an access panel under the fridge. Replaced the sale switch and I hooked everything back up. The fan turns on and the ignition does try to light now. The heat turns on for three seconds and turns off again. I do get hot air out of the exhaust for two or three seconds. I went through about 30 cycles with the same results. I can smell the propane at the exhaust pipe for three seconds the flame is lit, but it goes off after three seconds every time, not really sure what to do now.
Editing to add. The furnace is Dometic DFMD30121

[chuckster57]

Sounds a lot like the igniter/flame sensor. Gap is probably ok since it ignites, but the burner may be dirty. As cheap as they are I would simply replace and check the gap for ignition.

[sourdough]

Here is a rather lengthy post from another RV site that describes your problem sounds like. I'm linking it in the hopes it gives you some ideas. I would also note I've had a similar problem and it was the board.

https://www.rvnetwork.com/topic/1408...not-light-off/

[BigJohn1]

Thank you VERY MUCH for prompt advice, reading through the wealth of knowledge was extremely helpful.
I will admit that I definitely earn the moron of the day crown for this and I will wear it proudly. I actually replaced the sail switch twice. After I initially replaced it and put everything back together, the problem didn’t get fixed but I wasn’t 100% convinced that the aftermarket switch was good, it seemed flimsy. I cleaned out the original switch, which did have a little bit of dust and lint on it, and did the job over again…... Realizing that one of the connectors on the aftermarket switch was loose. Putting the stock switch back in is what gave me a spark for three seconds. What I failed to do after I did the job the second time, I didn’t replace the metal exhaust panel on the exterior, which the heat comes out of. I think that extra little bit of air coming in from outside was putting out the flame (there was zero wind). As soon as I put the plate back on, the furnace lit up and now works flawlessly. I figured it had to be something simple under the circumstances but that was just silly. An oversight on my part for sure, hopefully this helps someone else finding themselves in a similar situation.

[sourdough]

Just a note for future reference in regard to the exhaust (which I learned the hard way); they are very sensitive to ANY obstruction of airflow. I put a bug screen over one and it began the light up, run a few minutes, shutdown over and over. As I troubleshot the thing the only thing I had done was add the bug screen. I couldn't believe those little wires could cause that issue but it apparently either obstructed enough airflow or caused enough turbulence that the heat in the furnace went up and the high limit switch would cut it off every time - pulled off the screen and nary a problem afterward.

[jancoe1]

This sounds like a dometic furnace. If a back up wrench was not used to hold the gas valve in position when putting the line on the gas valve it will "tweak" the position of the burner assy on the other end of the gas valve. With the furnace pulled check if the gas valve looks square in the recessed area. If needed tweak the gas valve back into a square position. I have corrected alot of these newer dometic furnaces in New units at a dealership I recently left. Sail Switches are a big problem on these too. Make sure you replaced the plastic bracket that the sail switch is mounted to. There is a revision in the plastic mold. A notch has been made for the sail switch to fall into itself more.
Also this furnace needs its exhaust installed for use even if bench testing. Without it it's sucking the exhaust into the intake and choking itself without fresh air to burn.