02 impala displays code 0420,catalytic below efficency, component $60, bank1-8122 should be {8000 just had original catalytic replaced (obstructed)500 miles ago , what is Bank 1? ?????????

 

Bank 1 should be a general position locator if I remember correctly. On my '99 Silverado I was getting similar warnings, only mine would say something like "Bank 1, Sensor 2". I think Bank 1meant left or right side (exhaust manifold)and Sensor 2 meant post-catalyticconverter. I had 4 O2 sensors total IIRC.

Good luck.