Hi,
I have a natural gas furnace that is starting to give me grief. Often when I call for heat the motor and blower fan turn on and I get a blue induction flame, but the tubes won't ignite. At first I would turn off the furnace at the thermostat, wait for the fan to stop (which would sometimes take too long), then turn the system back on and it would start as normal. After a while the thermostat "trick" stopped working so I turned the furnace power off at the switch, had a look inside for loose wires (found none), then turned the switch back on. That seemed to fix the problem for a few hours but the problem reappeared. Now, when I can't get the burners to light, I go downstairs and flick the switch off for a few moments, then turn it back on and it works fine (again only for a couple of cycles but at least I can heat the house). I checked the continuity on all the easily accessible switches and they all seem fine with the exception of a disc located in the upper right hand corner of the upper panel, this has zero continuity. Also, I should mention, the blower fan sometimes doesn't turn off by turning the system off at the thermostat (sometimes it does). Any help is appreciated.

I had almost the same problem. They replaced the motor and everything worked perfectly.

Post brand/model# of furnace. Look for status LED behind clear viewer (if present), observe/note pattern. Check for service legend/label w/ codes.

I think I may have figured out what was wrong, thanks to the help of an HVAC guy at work. I cleaned the pilot light wires with some scotch bright and it's been working fine now for 24 hours. He told me that if there is a small build-up of residues on these wires it will interfere with a signal to the furnace telling it that there is a pilot flame. Without that signal the flame sensor won't allow the furnace to light. That would explain the hit and miss performance I was experiencing. Anyway, thanks aingyl and KC13. Hopefully the problem is solved.