Here is the definition of that code. "O2 Sensor CKT No Activity (Bank 1 Sensor 2)" What this means in laymans terms is that the rear O2 sensor is dead in your car. '96 and later vehicles are OBD II equiped, this means that they monitor certain components in your car and have readiness tests or monitors that they must pass in order to be fault free. Once the readiness tests are past and the monitors are said to be up the control unit monitors the systems and components that it is supposed to. If no faults are found then it will not post any codes and the lights stay off, in the event that one of the components fail or it does not pass one of the routine tests run by the ecu then a code is posted and the light comes on. In your case it has detected that the rear 02 sensor is not operating. The rear 02 will usually do one of 3 things, usually, it will either read correctly at around 4-500 millivolts, vary between .1-.9 volts (bad cat) or it will show no activity and be dead in the water, bad sensor. If you brought the car into my shop I would charge you and hour labor to pull the code and check the car out and half an hour labor to replace the sensor. 1.5 hours labor to inspect, diagnose, and replace the rear 02 sensor is a fair price. Volvo list on that part is 236.00 and another 25.00 for an adapter cable that they recommend. You can get the parts cheaper and do the work yourself but you will also have to provide your own warranty and actually get in there and do it yourself. Just a word to the wise, the connector on that sensor is kind of a pain in the ass and you will probably want to have an 02 sensor wrench to bust it loose.

Mark