You put way, WAY too much trust in today's sensors, electronics and software to believe that is always going to work. There are still a bunch of mechanical problems, that show up and no sensor or fault codes can pinpoint it to the exact point to get the RIGHT parts, each time.

The software and fault codes, might/will get you close to the electrical circuit or mechanical area, of a problem, most likely; but there can other issues, when "on site" that show up, that a mechanic may not have parts for, or even assumed that he may of needed such parts.

I heard those dang Salesmen,blowing smoke, about this same thing ever since electronics and computers were put on engines = They always say "The computer tells the technician exactly what the problem is" ; when they talk with customers. {They (salesmen) are so ignorant, and full bullcrap.} 

You get into a complex system, like some of these emission devices (DPF, ARD, etc. ) and all their thermocouplers (which don't have fault codes, when they read slightly incorrect, but those incorrect reading causes system failures, derates and shutdowns) then you start to understand the limitations of software and fault codes diagnostics, over the airways or some internet connection.