At same time spark electrode is firing (click, click, click)the gas valve opens.
Main flame doesn't light because burner slots are obstructed, spark electrode gap is wrong (S/B 1/8"), spark electrode is not directly over center of burner slots or spark is going to ground thru cracked ceramic
IF main flame lights then it is not engulfing the electrode so no 'proving' signal is getting generated back to circuit board
Flame 'ionization' generates a small milivolt signal that goes back to circuit board proving main flame has lit. No signal so circuit board drops DC Voltage to gas valve closing it.
Crack ceramic, electrode not engulfed in flame, electrode dirty, wire connections dirty/loose/corroded
