I received this message a short time ago. I took it to my mechanic, turns out to be an oxygen sensor. This sensor is in all newer vehicles, not just the Volvo. They tend to go out somewhere between 60 & 100k miles but can last longer. These are extremely important becuase they regulate the air fuel mixture for the injectors. If left unchecked the car can burn an extremely rich mixture and eventualy cause the catalytic converter to burn out. That would be a much more expensive proposition than replacing the sensor. Face it we own temermental European cars. The premium fuel requirement alone can break the bank. Maintenance is always going to run more than other cars. This sensor can run from $139 on various car parts websites, plus labor for installation. Problem is you need to find out which one, There should be at least two if not four on the car. I'd probably not take it back to the place that told you there was nothing wrong. They obviously don't know what they are doing. There are usually mechanics that either specilize in just Volvos or in just luxury cars that can properly diagnose the problem and determine which sensor has gone out.