I have a code that says one of my rear O2 sensors is ready to be replaced and I was wondering would that cause it to run rich? My local (non-rover) mechanic/friend told me it seems like its running rich. Are there any signs that would prove that, besides MPG that seems to be fine. My exhaust seems to be getting a little bit of black gunk buildup and I am beginning to wonder about that too.

 

A bad O2 sensor can make you run rich.

 

You need to replace all 4 02 sensors, clear the codes and see how it runs. By mileage all you 02's are weak and prone to poor readings.
Check with Atlantic British, I think they may have them on sale still for $59 each.