New sensors for trucks in the 01-04 range should read about 6.5 Ohms. Move the black wire on your multimeter to the ohm connector and set it to 200 ohms. Just unplug the connection on the top of the motor, to test without needing to remove the sensor. Test the two outside wires in each sensor side plug.
I was getting P0171 on my old Frontier, and that also was resolved by replacing the passenger side (Bank 1) heated oxygen sensor.
The complete list of items used in this project, and other projects, can be found in the Parts List below.