Klause is the master but I can tell you that failing O2 sensors trigger a variety of codes but all those codes have some relation to O2 sensors themselves. Those things are not cheap especially if you have one of the Volvos with the Denso ignition system, a minority of Volvos. So if the code means,"heater circuit for O2 Sensor failing" It is the sensor itself and not the wires that supply the heater part.

What type of engine is it, obviously not the 5 cylinder.

Running rich is usually a sign of a vacuum leak, then the front O2 sensors cannot do their thing properly.

If the S80 is used mainly as a short distance hauler, the engine doesn't get to warm up fully, take it on the highway occasionally to get the engine oil hot and clean out the piston chambers.

The S80s also have problems with the turbos failing, dumping a lot of oil into the exhaust. Look in the rear view mirror.