LEGENDA:
X2 has the following links : 11-10 , 9-8, 7-6 . Between the links are optocouplers that feel the power to the GAS VALVE and/or the overheat.
X7, 17 and 16 are switched together through a relays contact. This gives the consent to the Burner Board.
This schematic is a view of the whole circuit that controls ignition. It comprehends parts of the two boards and other components and adopts some semplifications.
The common problems concerns relays and capacitors on the Burner Board.
The cap. after R33, parallel to coil of “central” relais, was dried in my case, thus relay oscillated at 50Hz and damaged its contacts. Replacing the cap. (10uF/63V) and the relais (OMRON 48V), had the furnace back alive.