Please HELP - furnace issue
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