The problem with all main dealers these days seems to be that they've lost all the guys with experience and they teach new mechanics to rely too much on computer diagnostics. Every time I am waiting at the parts counter of a main dealer I hear customers being told "we've put it on the computer and it's fine".
Things haven't changed. To fault find a car you still need to understand how all the systems work, just like you needed to in the days of distributors and carburettors. You also need to understand the limitations of the diagnostic aids you have, and understand why they're telling you something, not what they're telling you.
Sadly, most main dealer mechanics will replace anything mentioned by the diagnostic software with no further thought. Once they've run out of trouble codes and there's still a fault they assume it's the ECU - because it's not reporting a problem when there is one!

Kevin