It's my experience that very few oxygen sensor problems are the sensors them selves, especially if they don't have 120k on them. There are numerous fault codes for O2 sensors. Proper diagnosis requires testing based upon what protocol is failed. All the parameters that are monitored by the control unit can be monitored by testing. Min and max voltages, rich lean switching time, lean rich switching time, cycles per second. Sometimes the time to the first switch is monitored and fails due to the engine not being heated properly.
Check your code and follow the diagnostics based upon the number.
Bosch Master, ASE Master, L1
33 years MB technician