usually lean is not a sensor. f had pre cat lean would look at engine but being post cat lean bank 1 would look between front and rear o2 sensor (on indicated bank) for exhaust leak, or cat is having issue converting gasses, or rear o2 is stuck lean (false reading like sensor is done, wires shorted, open circuit, contaminated...). I would eliminate by checking for exhaust leaks closely (not familiar with 3.0 setup) start by having friend plug tailpipes with rags firmly while you listen for leaks. If no leaks and is only post cat fault would watch live data if possible and see if sensor is working, what is reading exactly, test by unplug and watch for change, and introduce an actual vacuum leak at engine watch for change. do visual of wires to sensor and if no issues found guess at rear sensor for bank indicated (cheaper than a cat).
