Diagnostic codes are needed to troubleshoot problems as cars get more
complex. Unfortunately auto manufacturers take the position that
diagnostic codes are proprietary and only their dealers may have access
to codes' meanings.
This leaves us car owners and all independent repair shops in the cold,
without ability to diagnose problems and grants, defacto, dealers a
monopoly on some repair work.
There's a bill in Congress, regardless of whatever else you may think of
Congress, to create an "open source" system for diagnostic codes. This
website has additional information, I urge you to support the bill with
an email to your legislator.