How canI fix my sarnier duval boiler?

we have a saurnier duval isofast f35e boiler. Engineer fitted a pcb because following elacement of fuse warning triangle was flashing but no fault showing. After PCB fitted, fault ff07 showing - domestic hot water thermistor. Engineer fitteed two thermistors - now showng fault ff20 - communication. System hasnt fired up at all, can anyone give any advice?