O2 sesor/cat issue

[r lane]

09 328i that has thrown O2 sensor codes upstream, bank 1. O2 emission check 2C31. On actual values, it was indicated that there was also exhaust flow issues. We have replaced both up and downstream sensors as well as a cat from the dealer and the code still persists. Any knowledge in this department would be immensely helpful as we have much time and angst in this effort and going nowhere. Thanks, Bob