Without known the exact code you got, it is impossible for us to gauge if this is an issue or not.
For example, if its the heater circuit, then you will probably be OK.
If the sensor is measuring rich the whole time, then you will probably not be OK (the catalytic converter could overheat and internally melt/get plugged).
if you have a bad o2 sensor sending a lean signal to the ecu for one side of the engine, then yes is will drive the ECU to create a rich condition to compensate, fouling the spark plugs on that side. an exhaust leak can cause an o2 sensor to read lean, or possibly a short or bare wire in the harness, or defective o2 sensor. or perhaps it really does have a ...