Furnace won't stay lit
This is a Ducane 80% furnace 75kBTU. It does have an LED indication on the circuit board which only gives two indication during the problem I described above. Slow hartbeat when power is on, waiting for call, then fast heartbeat when there is demand for heat. Again, normal according to the error codes shown on the inside of the panel. I have noticed that the heartbeat "skips" a beat when the voltage drops to the gas valve, but does not revert to any of the fault codes shown on the panel; still fast heartbeat. I have seen the other codes (4-pulse of 5-pulse etc.) when conduction tests of the limit and vacuum sesors though. It almost seems like the system needs to warm up before the problem occurs because it runs normally for the first several minutes before it cycles off and on. I may look at the flame sensor again to try to see if there may be a hairline fracture in the ceramic or something like that, and also inspect the solder joints on the back of the control panel. Beyond that I would love to hear any more suggestions - Thanks!