RE: o2 sensors
Bank 1 or Bank 2, when you pull codes it will say one of those, Bank 1 is generally on the #1 cylinder side of the engine on most cars. On OBDII cars it will say B1O2 for Bank 1 O2 sensor 2 which would be the rear o2 on the passenger side of our cars. Like he said, replace both or you may get "lazy" o2 signals.