A few weeks back, my '06 X3 3.0i service engine light came on. Codes read were P0305, P0174 and P0171. I've read a lot in the forums about possible causes. A few days ago, I reset the codes and they haven't come back, so I am reluctant to spend time taking things apart to look for vacuum leak or other issues I've seen in the forums. Is there some possibility that this was a one-time fluke, perhaps a bad tank of gasoline or something like that? I also thought I read that it could be dirty MAF as a result of oil from a K&N air filter, which I have had one for about a year now.