by how » Sat Jan 21, 2012 12:34 am
Usually between the pilot coming on and the main gas valve opening is a proving system that must verify that the pilot is in fact lit.
A faulty or dirty flame sensor (or loose ground wire for the sensor) is the most common cause for the furnace not continuing on to the powering of the gas valve.
If the gas valve is receiving 24V and not opening then the gas valve is suspect.