P0050 AUDI

Question

i have an Audi A4 3.0 2004... i got check engine light, so took the car to autozone to have it scanned. I got error P0050, which is bank 2 sensor 1 for the oxygen sensor. I got them changed, pre cat and i still get the error.. the mechanic is tilling me that i need to change the back 2 as well, but i am getting error p0050 which is pre cat. what else could be wrong.
i have new sensors on and still getting the error. what can you suggest