[jsb5717]

Originally Posted by R.S.O'Donnell

Ours was doin the same. It was acting like the flame sensor/ignitor wasn't detecting a flame until it had gone through several tries. I attempted to remove the ignitor but too much had to be removed to get it out with the time I had so I put the screws back in. Inexplicably it worked flawlessly after that. It's been working perfectly ever since.

This is the best explanation I've found on how they work:

Interesting, but is that an RV furnace? Looks residential to me. Unless there is a recent change the igniter would be the spark type that you can hear clicking as it attempts to light. That same scenario, though, was happening to us. ie.: actually igniting but turning right back off. For us it turned out to be the propane regulator, which wasn't supplying a flow adequate for the high demand of an RV furnace. New regulator solved the problem.

Question for the OP is whether the furnace is failing before or after the igniter attempts to fire. That will help to trouble shoot if the issue is pre or post ignition.