First of all, be SURE that it's your O2 sensor. Sometimes another bad component (for example, a cat. converter) can cause bad O2 sensor readings, and throw an O2 sensor code. Whether or not that's the case in your car, I dont know, you'd really need an OBD-2 scanner to see the output of the sensor to really determine what's going on. Then again, it could be as simple as your O2 sensor wiring coming undone or loose, and unplug/plug the connector could solve it. The problem with doing it yourself if that you can end up replacing things that dont need replacing, etc. Some shops work that way too, unfortunately. If you're really that poor, perhaps you should sell the (expensive car that you somehow were able to purchase) 1999 S70 and get something more economical though, things can get expensive fast. Good luck.