Huge thanks for this, same problem. My gas engineer tried to persuade me to replace the boiler for £2k saying a new PCB might come to as much as £400 to replace.
I wasn't convinced by this and fortunately checked for parts in local supplier for boiler spares, £80 ex VAT. Then I came across this post and realised I might be being taken for a ride! I noted at the same time that most parts for this boiler are fairly cheap to obtain, so if you have an engineer trying to charge you top dollar for genuine parts, check them first on the internet, most places will deliver within 24 hours if you can't find a supplier locally and so you could save yourself a fortune!
Ariston upgraded the spec of the PCB if you have a 2000-2004 model, so there may well have been problems with the original board, certainly the new one seems to make the boiler run much smoother.
The main PCB is very simple to fix yourself, just treat it with respect and care. Please remember to turn off the boiler electricity supply with isolator switch before doing anything. Unscrew the front, and remove cover to reveal PCBs, unplug the five connecting plugs (remember which socket the earth goes to and which the black lead goes to - I usually photograph these things with my phone before unplugging so I can remember what goes where). Unscrew the broken board and screw new one into place, and reconnect wires. Turn boiler on and it should start again fine, mine did anyway.
Once again my thanks to those who posted above - this is what the internet is for!
