On a Nissan Maxima which O2 sensor is Bank 2 Sensor 1

Bank 1 Sensor 1: Located on rear exhaust manifold pipe nearest the firewall. Fairly easy to remove with the right tools. Clip located on left side of engine compartment.
Bank 1 Sensor 2: Located just behind the Catalytic Converter. Very easy to remove and clips on just inside rubber grommet.
Bank 2 Sensor 1: Located on front exhaust manifold pipe nearest the radiator. Hard to reach without specific tools. Clip located in front of engine compartment.

Answer

Bank 1 = both front firewall-side sensor (1) AND rear sensor (2). Bank 2 = front radiator-side sensor (1). Bank one refers to the section of the manifold that vents exhaust from the cylinder bank including cylinder #1...bank two = from cylinder #2 and its comrades.
I hope this has answered any question about the O2 sensor on a Nissan Maxima