Vehicle is 1999 ML 320 with 107,900 miles on it.

Vehicle runs and idles fine no different than before check engine light came on.

Code reader shows P0156 which is Bank 2 Sensor 2 malfunction. I believe that refers to the driver's side O2 sensor after the CAT.

Questions
Should I replace that O2 sensor? And if so why when the vehicle is running fine. Will any harm be done to the vehicle to run it with this fault?

Edited:

I don't want to replace an O2 sensor just because a computer told the check engine light to come on unless it's necessary. For all I know the O2 sensor may be barely out of the designated parameters but still work just fine.