[JimKrieger]

Had the exact same problem. For me it was a bad igniter. The fan ran for about 30 seconds and then, because a thermistor sensed no heat, the circuit board shut the furnace off. In addition to the shut off symptom during each of the three 30 second ignition attempts, I would notice no heat coming from the exhaust pipe, but did smell propane there indicating a gas supply was present. My unit has no outside furnace access, so had to move the furnace about 30º leaving the flexible gas line intact, to get at the igniter. Igniter requires a 1/8" gap which can easily be checked with a 1/8" drill bit.