Here is the way I understand it, there are a bunch of odb2 codes that are the same cross all platforms (cars and trucks and so on) Based on the ODB2 standard. Then there are a bunch of codes that each maker has that are specific to the vehicle or make. The cheep odb2 scanners will only read the standard codes. If you want all the specific codes you need something like auto enginuity, and they aint cheap… I have read about people on here that have a CEL and the cheap code readers (like the one at autozone) say there are no codes found.
My bro in law has a ford focus that has a CEL and the autozone reader tells you its something like a PO 0000 which does not exist.