Ignition problem VAILLANT 828 turbo + lpg 10 years old?

hi i have a vaillant 828 comi boiler. the boiler has spoted working. the green light stays on and no sigh of the fan working. dianostics say is the fan or the air presure switch or curcit board. i have change the air presure switch no difrence still not working. I checked if the fan is getting power very strange the brown live switch connected to the fan is live when tested but then cliped into the fan no live power . this realy confused me how can one wire have power then not? i reset the boiler many of times and once or twice the fan has worked them stopped. the fan seems ok but could some on please help me.

M.Sohail
May 2011