Fault code 'A02' with my Ferroli Optimax HE Boiler?

I have a continued problem with my boiler that just won't go away! About once a week the heating & hot water go off & code 'A02' appears on the boiler display. After pressing the reset button it fires up and all is well until it happens again. I've had a Ferroli engineer to have a look at it twice now, first time he placed it in test mode which took the temperature right up to clear any oil, debris etc. The problem continued & 2nd visit he replaced the circuit board. The problem hasn't disappeared. Before I call them for another visit (& possible charge) I'd be grateful of any suggestions that I can put to the engineer that might help resolve this issue. many thanks