There are left bank and right bank primary sensors (up-stream before the cats). There are 4 sensors total 2 pre-cat and 2 post-cat.
The OBD II code will provide you with a "Bank 1 Sensor 1" or
"Bank 2 Sensor 1" location. Sensor codes do not always mean the sensor is bad. So sensor testing or the other engine codes might point you in the right direction when trouble-shooting. How did you decide to replace the sensors?
1. In class I have had rabbits eat the sensor wires on school lab vehicles parked outside. Sometimes you can repair the wire using good soldering methods. Sometimes they eat the wire right at the sensor creating a un-repairable sensor.
2. If you have a lean bank of cylinders the O2 sensor code might indicate there is something like a vacuum leak that replacing the sensor will not fix.