At first our furnace heat wasn't coming on, we called our home warranty company and they sent out a HVAC guy. He replaced the inducer motor and the pressure switch. Then it still wasn't coming on properly so he said it was the exhaust or flu pipe that was blocked and we had to call a plumber to come and check on it. We have since had 2 plumbers check the pipe and nothing was in it. The last time the HVAC guy came he showed us that the furnace would come on, or light, when the flu pipe was off but wouldn't light when the flu pipe was connected, that's why he concluded there was stoppage in the pipe. If we take the pipe off and start up the furnace, then connect the pipe while it's running it will stay on. The HVAC guy has already been out to my house about 5 times now and he obviously doesn't know what else can be causing it.