And that's a flaw in the design of modern vehicles. The vague error messages give mechanics lots of leeway on screwing over people, much like computer repair shops will charge exorbitantly to the person who doesn't understand computers.

Transparent error messages are more useful, and gatekeeping drivers with a connector and a list of codes should be considered unethical. Too many industries are trying to sell you something, but not let you make it yours or let you understand it. Before long, the value proposition will fall.