For this code you don't replace any o2 sensor. the issue is with the adaptation values for the dme.your dealer will have to do the following since they have the proper diagnostic tools.
tell them to check/clear the code.
reset the adaptation values.
do a "quick delete" before disconnecting communication between diagnostic equipment and car.
disconnect the battery from the car.
do a battery reset- touch both connectors that go to the battery posts with with a wrench for a minute.
let battery sit disconnected for 1/2 hour.
check to see if code is gone.
if not then do it all over again and let the battery sit longer. maybe an hour.
the dme has to reset the amount of fuel it delivers to the engine, which is causing you "rich"code.
Source: North American Motoring Forums