Oxygen Sensor

1996 Grand Caravan SE 3.3L The check engine soon light is on and I am getting o2 sensor code 21. I have disconected the batt twice to clear codes to be sure and after some time i keep getting same code back.
Have any of you had this and which one seems to go out first the one before converter or the one after converter. I don't really want to replace both at one time but without getting exact code to which one it is it will be only a guess to which one is going out.

The O2 sensor that usually goes is the one in front of the CAT because it's nearest to the exhaust maniforld (and is subjected to the hottest temperatures).
Also, should both O2 sensors fail, you will see code 21 flashed twice in the list of codes before the end of the list - code 55.