Almost forgot - the online diagnostic manuals will be a huge help, once you learn how to navigate them. And, they include the DTC's which may be easier than poking through the big PDF... or not:
http://manual.startekinfo.com/manual/JSP/sitemap.jsp

Jono is right, you could chase ghost codes if the wiring is hosed. Live data is a huuuge help but you need to know what you're looking at. On the M120 this can be a bit easier as you can compare left bank readings to right bank and see if anything is drastically different.