The link below is to a list of OBD-II codes for the 2005+ Xterra. It also references the section/page in the Service Manual for more information (you must already have the service manual).
http://jbeamphotos.com/users/jbeam/offroad/05Xterra/05Xterra_OBDII_Codes.htm
http://jbeamphotos.com/users/jbeam/offroad/05Xterra/05Xterra_OBDII_Codes.htm