I Like your level of difficulty chart.
Don't assume a code means a part change .. that is more often not the case.
These codes simply mean that a fault has been detected on that system that did not pass the self test. These are Emissions codes only.
An example on a 104 would be a code 5.. very seldom does this code get rectified by changing the EGR valve, but rather clearing out the intake egr tube port at the the engine...hardly an EASY job..
Whenever one wants a clearer picture for diagnostics on these HFM systems, it is best to see what codes are coupled up with the emissions codes from the actual Engine Management modules.