are u using a wideband afr gauge or a narrow band gauge also is it a heated o2 if its a narrow band.
reason i ask is if its a narrow band o2 they are only accurate around 14.7 afr at 15.5 afr the thing can read dead lean, same goes for the opposite direction.
also if ur using a narrow band and its a non heated type at idle the sensor may be droping below operating temp witch will also cause a lean reading.
if u are using a single wire o2 sensor i suggest going to a 3 or 4 wire heated o2 both are the same the only diff between the 2 is the 4 wire has its own ground wire so it dosent rely on getting ground from being screwed into the ex system
also narrow bands should not be relied apon to much for anything other then wot