worcester greenstar 30 si think its the pcb?

Can anyone shed any light. Gone to a customer boiler in lockout indicating ignition fault. Reset boiler pump comes on but fan doesn't then goes to lockout, so I suspect fan however tried again, this time just after pump comes on and before boiler goes to lockout I blow down the air pressure switch tube and find fan to come on boiler then runs fine and modulates perfectly until it switches off, then back to square one. The pressure switch is definetley OK and not sticking in open position before firing. Anyone got any ideas I think PCB personally but you know what its like returning them. Any help will be appreciated. Thanks brad