Q: OBD2 Codes and Diagnostic (follow up question)

The data you are seeking, documenting relative frequency of failure of all the various powertrain components, is maintained by the scan tool manufacturers examples of which are Actron and Innova. These companies, as well as other scan tool makers, compile the most frequently encountered codes and then additionally report the most common fixes (i.e., the actual repairs) in order to aid mechanics. Indeed they sell this data, that is the advice as to actual repairs based on reported codes, through various subscription services. YourMechanic gathers data as well and you can address an inquiry to partners@yourmechanic.com with a specific data request. All of the data, and studies, that I have seen thus far, or indeed could contemplate, is not assembled at global, industry level but rather represents lots of separate, disparate investigations. Many are quite good and I referred you to that 2009 Illinois EPA study for instance. But, once you look into this, you’ll end attempting to collate or, mix and match, a lot of "incompatible studies". You should further limit the scope of your inquiry as otherwise you are going to have an unmanageable project on your hands.