Thanks KiwiC.
I have been asked advice on what to do with failures when in the jumpseat, and my advice is always "whatever the QRH says" then after that is actioned I have discussed the system with crew. But not for troubleshooting, more for crew information
That is what I would hope/expect to be the case.
BTW: When I mentioned not debugging on a revenue flight I meant taking action, not observing and possibly correlating data not recorded such as actual displays etc although as you point out in most cases it probably just confirms the reported fault.
I suspect there may be times when observing the fault first hand may clarify things since the write up could be ambiguous,vague or operates as designed but not as desired/expected.