66K is somewhat early but stuff does fail. ON the 850's, volvo says to change them at 100K so I'd guess it would probably be similiar on the s60. As the front sensor helps set your fuel mixture, the engine generally runs richer and richer as time goes by. You'll see your mpg start to drop off usually before the CE light comes on. It's not a good idea to run them very long once you know the sensor is bad as the overly rich mixture is real hard on your catalytic converter. As me how I know.

bl