96 and newer contours with V6 have 4 O2 sensors. (2 on each Cyl bank) The P0420 code indicates that there isn't enough of a difference between the switching rates of the upstream and downstream sensor for bank 1. (By the firewall) If both the upstream and downstream sensors have been replaced then your pre-cat may be dying. The pre-cats are covered by the 80K mile emmisions warranty if that helps.
