I am constantly replacing the oxygen sensors after about 30,000 miles each time. The problem usually begins with a bad start where the car mis-fires. This leads to a rich mixture for a while and then the sensors fail. When I replace the sensors the problem goes away for another 30k unless I make too many short trips before the car actually heats up. I feel like I am only treating the symptom. Anybody got any idea why this is happening? A couple of Volvo mechanics I went to were stumped. They just wanted to keep replacing sensors at $500 a whack.

Thanks for your help.

Pete