Mine was recently found to be shorting or something...essentially
inoperative, but sending a 'wrong' signal that caused my Digifant 92 jetta
motored 84 to run badly.  I unplugged it and it runs fine.  Plugged it back
and it ran fine until it warmed up...then bagan stumbling and running super
rich and badly again..
   With lots of list help, I am pretty sure there was an issue with the O2
sensor, so I un plugged it and I have driven the van about without the O2
sensor...it runs just fine, but I think I am getting poor fuel economy and
there is a sooty deposit on the bumper by the exhaust tip...Anyhow, my new
Bosch single wire sensor came from Van Cafe and I just installed that,
started the van and ran it till it was fully warm....without any stumbling
or missing...There is no cat in this van, by the way...But here is the odd
thing...I can't really tell any difference in the way it runs....It ran
'perfect' without the sensor hooked up and it now runs perfect again with
the new one installed...But I would like to know if I now have fixed the
whole problem or if I actually still have a problem between the O2 sensor
and the ECU......and now maybe the wires aren't really connected and the
sensor is still in op...

 so my new question is:  Is there a Backyard Mechanic way to tell if the O2
sensor is now functioning as it should be?  I know I can probably count on
better gas mileage again, if it's working, so I can go fill and see how
crappy it was doing without the sensor plugged in, then run a tank full and
see if I am back to my normal 23mpg....But, is there some trick way, like
unplug a vac line, or one of the other sensors and get some indication that
the new O2 sensor is or is not functioning?  I wish I had one of those
exhaust gas readers, but no luck there..
  Thanks,,Don Hanson