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.