Does anyone know where to go to get more information on what individual OBDII codes mean? The codes are often quite brief, like P0171 "System too lean (bank 1)". This happens to be the current code on a family member's 2001 Passat 1.8T (4 cyl. - only one bank) - which is running fine - but the code and check engine light keep coming back. It would seem like maybe a bad O2 sensor but there must be a dozen codes for O2 sensor - so why wouldn't a bad O2 sensor give an O2 sensor code?

It seems like somewhere there should be an analysis of the probable causes are for each of the codes.