CEL - Main catalyst bank 2 below efficiency threshold
 

Need some help. I have an 01 A6 4.2, got a CEL for "Main catalyst bank 2 below efficiency threshold" The dealer wants to install a newcat in the drivers side for $1,600. There are no other codes, so is it possiblethat I have a bad o2 sensor? If so pre orpost cat? I would rather change the o2 sensor for less $$ Thanks.