Q: Looking for info about most common OBD2 Codes

There are two (at least) quite distinct issues in regard to your general request: The first is, once a check engine light illuminates, what are the most "frequent" codes that are stored and thence downloaded, that is what does THAT distribution look like?; but the second issue, and probably the more meaningful one (in terms of engineering, economics, market research, etc.) is, what are the most common "problems" that are actually in evidence (found) upon doing the actual diagnostic? A key point is diagnostic trouble codes merely provide clues in the actual process of repairing an automobile. Codes do NOT conclusively or authoritatively identify failed parts. As such, codes mean little (in real life) unless accompanied by an actual diagnostic. The automobile manufacturers, as well as the scan tool manufacturers, DO absolutely have BOTH categories of data that you are looking for (i.e., both categories that I just described) but the difficulty of course would be obtaining the data from them. There are many other entitles, though, that collect this "type" of data that you are referring to and just to give you merely one example (there are MANY), in 2009 the Illinois Environmental Protection Agency looked at the records of 1.67 million inspections in the state to determine, inter alia, what the most common trouble codes were. A list was compiled and published by the Agency. If you let us know more specifically the GOAL of your research, that is the questions you are trying to answer or the market estimates you are trying to create, we will be happy to refer you to the most appropriate databases, vendors, standards setting organizations, and/or academic literature that will address your research goals. If you have further questions or concerns, please do not hesitate to re-contact YourMechanic as we are always here to help you.