I have a 99 XC70 and have had this really annoying problem with it. The exhaust smells like rotten eggs. I have switched gas and gas stations many times, driven the car in 2nd gear at highway speeds for more than 30 minutes, have the cat. replaced (under the 80k emissions warranty) and also a new front o2 sensor. Still the smell persists. Any suggestions? My wife wants it fixed and there is not much left to replace, other than the whole car, which is hard to get Volvo to do a buyback which makes it strainful for me since I work at a Volvo Dealership.