The propane tank is at 60%. It was my thought too, that it sure seems like an oxygen issue, since the burners light with the door open. I watched the flame sequence with the door closed and noticed it lit the first and second burners yesterday, but then stopped, and shut down. So I turned it off and tried to feel for any build up/debris on the opposite end of the burner from the gas jets (which were clean), but I could barely get my small hand in there. So I carefully took a toothbrush and tried to clean up anything that might be blocking the flow of gas from burner pipe to burner pipe. That worked, as the next time I turned it on with the door on the chamber all burners fired normally. This morning at the call for heat, all worked great. Just now, when it kicked back in, no heat again.
I did check the vent pipe yesterday, all was clear from outside and the 20 feet to the bend in the pipe to the furnace itself. Inside the basement, all of the pipe joints were glued, so not easy to remove and check at that location. But from outside, I can feel the draw of air into the furnace at the pipe opening. So, Freon, you are probably correct in that I should focus on the inducer fan.
One more thought, could ANY of this have anything to do with the programable thermostat? Perhaps its faulty? I just changed the batteries in it, and turned off the program to run manually during all of this furnace issue, but it would seem odd to have that be the problem.