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.

too lean gives 51
too rich gives 52
neither rich nor lean condition gives 21 which is ambiguous. does it mean sensor should be replaced? i believe so but i'm not sure. manual is confusing here.
using silicone sealants can destroy oxygen sensor.

I believe you can get them for less than $50 with the factory plug ends so the worst it is going to be is $100 plus a couple of hours of time. Not bad to solve the problem in my opinion. I have paid over $100 for a generic (no plug end, slice and splice) for other cars.

If your driving a 96' with origional O2 sensors you might as well change both of them now 'cause they are long over due. I have two 96's and I have had to replace four sensors.