I have a York furnace that the igniter glows, small blower, the gas comes on and I get blue flames for about 3 seconds. The cycle repeats. The main blower does not turn on. I checked to make sure the exhuast was clear and there's no error codes on module. Also, the main blower works in 'fan on' mode.

The flame sensor needs cleaned with steel wool. Opposite side of the igntor

Thanks! This worked for me. The sensor is an easily replacable item. Since the unit is 16 years old, it's worth having a new one on hand.

Nope, I have never seen one go bad. Just needs cleaned from time to time.