what are bank 1 & 2 referring to on 98 nissan maxima?

Question

Faulty Rear Heated Oxygen Sensor Bank 1 (P0139)
what are bank 1 & 2 referring to on 98 nissan maxima?
Where is it located?
Car runs fine but won't smog. What is most likely cause?

Answer 1

Images with the locations of the sensors and banks here: http://nissanhelp.com/diy/maxima/projects/1996_2003_nissan_maxima_o2_sensor_location.php

If you Maxima fails the smog test and has the P0139 code, then the most likely cause of the problem is the O2 sensor.