wont go over 2300rpms p0138 h02s12

Question

P0138 NISSAN - after hard acceleration 2x's, it won't go over 2300rpms, code is P0138 ,(H02S12) is this bank1 sensor2 rear? 2002 maxima SE, Also front motor mount is shot. Won't start when hot, after cool down it starts, but still only 2300 rpms. Where is this sensor located on my engine? Thanks David N.O., LA