I think that one that is operating properly starts to 'wear out', and fails gracefully, thereby not alerting the ECU to it's sudden demise. Our two '96s are over 140k and 150k, and niether has thrown a code, but with an item that's made to last <100k, surely they've gone by now. On my '96, I replaced it in the 120s, and the MPG jumped right up. My '92 had the 'Maint. Reqd' light, which goes on in the 80s. Replaced O2 at about 130k, MPG highway went from 15~16 to over 23. Paid for itself quickly.
We had a '98 ZJ, O2 went at about 65k, threw a code. Replaced it, and id did better, but light kept coming back every few weeks. Got to where we kept the scanner in the car, and whenever the light came on, cure it in about three minutes. Dealer said the computer needed to be re-flashed, to recognize the new piece, but wanted over $100 to 'diagnose' it. I reminded them that emissions were warranted for 8 years/80k miles, and I was told they may refund it. They lost a customer for life.