Heya Tdag,
What kind of vehicle are you thinking of running diagnostics on? The newer vehicles have a port, where mechanics can plug into, which will diagnose the problems, and inform of poor operation related to specific functions. Manufacturers install their own set of on-board processors, which speak their own language, so to speak. Understanding the specific logic of the exclusive on board processor is the key to the issue. The logic may vary, from manufacturer and model. I think the decoding is left to the hackers, or mechanics who divulge this information. Of course, you can understand that the specific manufacturers don't want just anyone to have access to the information that is provided by the on board processor. It boils down to another marketing tactic.
There are some sites on the internet, which describe of interfacing with specific vehicle models. Hopefully, you can find a site which pertains to your model?