On a call for heat your thermostat will close a contact between the R terminal and the W terminal on your main circuit board. This energizes the logic board. If all limit switches and other optional safeties are closed the draft inducer will start and run for a predetermined amount of time. The draft proving switch should close as soon as the inducer is up to speed.
After a time delay, called the purge cycle, the hot surface ignitor will began to heat up and glow. When it reaches proper temperature and is glowing orange, the main gas valve will energize and the gas will ignite. The flame must travel across the burners to the flame rod on the opposite side. The flame rod is the part that tells the logic board there is a flame. It does this by a process called flame rectification. If there is no rectification the logic panel will lock out based on a time limit.
The process of operation of the logic board is that alternating voltage is applied between the flame rod and a grounding source. The flame rod is inserted into the flame. The grounding source is connected to the burners so it actually touches the base of the flame. The ions in the flame provide a high resistance current path to ground. Due to the fact that the base of the flame is larger than the flame rod, more electrons will flow in one direction than the other. (For proper rectification the grounding surface must be at least 4 times greater than the flame rod surface.) This results in a small DC current flow. The logic board senses this and allows the flame to continue. If the flame rod is shorted to ground there will be a large a/c current flow to ground and the flame will not prove. If there is no flame present there will be no current flow and the flame will not prove.
Nuisance lockouts on flame failure can be caused by many things such as loose ground, dirty flame rod, oxidized flame rod, cracked porcelain insulator on flame rod, flame not carrying over burner properly, dirty burners, low gas pressure, hot surface ignitor failing, improper burner adjustment, bad heat exhanger.