P0171 has nothing to do with a failed or failing bank one (firewall bank) O2 sensor. Failing or failed sensors will set other codes. 99% of the time a 171 code (or 174 code for bank 2) is due to a vacuum leak. If you dont have an active 174 or pending 174 you likely have a vacuum leak that only effects bank 1. It doesnt take much of a vac leak to set 171 / 174 codes.