FWIW, I once replaced an O2 sensor just because it was time and I was
putting a new exhaust system on anyway.  The engine was running fine.

After putting in the new stuff I went for a ride.  The engine ran badly
especially at highway speeds.  I disconnected the O2 sensor and it ran
well with it disconnected.  It turns out I got a bad sensor right out of
the box.  After putting on yet another new sensor all was well.  I tried
the (used once) sensor on another Vanagon and it caused that engine to
run badly, too.  Not all that is new is good.

Mike
Houlton, Maine/Glade Spring, Virginia