Here at Royson's Automotive in Blythewood we often do diagnostics and sometimes the problem is obvious after hooking the car up to the computer and reading the codes.  However, sometimes it isn't obvious during a test drive or after hooking it up to all those computer programs!  You know there is a problem and a Check Engine Light is on with the fleet customer telling you the car has been having issues starting and the light has been going off and on depending on several factors.  No pattern, no simple answer but the problem becomes obvious when you start following the "threads of the issue" thanks to all that ASE Technician training.