Oh..Ok
That is the one we make here..

http://members.aol.com/ajdalton7/ledtool.jpg

Just gets you a direction to a problem

But , a good reader is $$$ , so I would stay with that for now.
Codes are not a diagnosis, but an indicator... if you don't know how each system inter-relates and functions, they do not tell you the problem/cause. They just report what the modules are seeing out of norm.