Why pay for diagnostic?

Do you understand the difference between diagnosing a problem with the vehicle vs. checking a code in the computer.  Simply checking the code in the computer can give a general idea of what the issue is, but if the root cause is not from a faulty part there are no additional steps you can take without running a full diagnostic.  Doing that could include testing electrical systems or tracing wires to find the root cause.  Running the vehicle to test multiple scenarios and to reproduce issues.  This can take a great deal of time to determine the exact issue to insure it is repaired correctly.