I bought all new sensors for my 1998 Chevy Suburban
on Ebay in March of 2007.  The sensors are suppose
to be Borg Warner brand, but they arrived in plain
white boxes with not BWD markings on the boxes or
the sensors.  The two back sensors (the ones after
the cats) have now both failed; one failed over
the Summer, and one just recently.  The two
front sensors seem to still be okay.  When the
first sensor died, I just put one of the old sensors
back in; the error code did not come back.
I asked the seller if they had reports of problems
with these sensors, and they responded no and that
I most likely have a lean or rich fuel problem
burning them out.  My questions are:

1)  Can an engine problem cause O2 sensors to "burn out"?
2)  If #1 is so, what is the most common problem to kill them?
3)  If #1 is so, why have only the back sensors died?
4)  Are Borg-Warner brand sensors unmarked?
5)  What is the best brand of sensor?
6)  What brand was originally used by the manufacturer?
   (If AC delco, is Delphi the same thing?)

The history behind this started with a repeating
P0430 error code that would go away when replacing
the bank-2 cat, but would come back after a month
because the bank-2 cat dies.  I believe this problem
has been fixed by replacing the fuel injection spider.
The car was driven for half a year before the sensors
were replaced; if the engine problem killed the sensors,
then why does it seem my original sensors are still good?

Thanks for any insight,

John Hermann