Cannot offer any advice but can empathise with you. Our last house had a Bosch Greenstar service boiler which developed an intermittent fault causing it to trip out usually on start up and very rarely in mid session. At the time we had a service and maintenance contract with Eon. I told the first service guy it had all the hallmarks of an electronic issue, but no. So said the procession of other guys who turned up every two weeks to replace some component or other. Eventually having replaced just about everything except the heat exchanger and the PCB an engineer turned up and announced after much head scratching and as a result of a process of elimination it had to be the PCB. This was duly replaced and problem finally solved. Fortunetly I was retired at the time but would have been mightily annoyed at the time off work the repair would have required not to mention the inconvenience of the heating being off. The only positive from it was that we virtually ended up with a complete new boiler which the last service engineer grudgingly pointed out to me.

There got it of my chest!

Richard