[77newell]

It depends on exactly what is happening. The sequence is the blower starts and runs for a set time, then the ignitor starts arcing and the gas valve snaps open (you can hear it), then a flame detector checks for flame which if it finds it within a set time it allows the gas valve to remain open until the thermostat signals for the gas valve to close, then the blower continues for a set time. If the flame detector doesn't signal that it sees flame soon enough the gas valve closes. If the overtemperature detector detects high temperature the gas valve shuts. If the airflow detector for the blower doesn't report sufficient airflow the gas valve closes.

So if the blower starts, runs for a while then stops for awhile before doing this again it could be any one of several things. To figure out which it is you have to get up close to it to see where in the cycle described above things come to a halt. Is the ignitor arcing, can you feel and hear the gas valve open, do you hear the flame ignite, can you see flame, etc. I had an instance when mud daubers had filled the blower scroll to the point where the airflow detector wouldn't actuate so the gas valve never opened and the ignitor never arced. It sounds complicated but if you get up close and personal with the furnace you will pretty quickly figure it out.