100K is when volvo recommends changing them but you see a lot of posts on the BB that shows they can and do fail earlier. Using off brand, no name convenience store gas over time may be a factor as it lacks the cleaning additives of major brands. Go to volvospeed.com (Bay 13) and go thru the vacuum leak detection section first to make sure you don't have a leak that is fooling your sensor into enriching the mixture. I forget the exact voltage but the front sensor will read ~ .3-.8v and will fluxuate rapidly and constantly in that range. You will need a digital VOM to detect the voltage as it is so small. If you have one, disconnect the front O2 sensor connector up in the engine bay and connect your VOM to the black signal lead from the sensor and ground it. Start the engine and see if the above fluxuating voltage is present. ALso check the 200/700 FAQ section as they have a lot of info on troubleshooting sensors.