Code 21

I have a 1996 Plymouth Grand Voyager SE with a 3.3L. The check engine
light has intermittently come on. Some times it will remain lit for
the entire time that the engine is running, other times it won't light
for a month. The key on-off method displays a code 21, which according
to my manual is the O2 sensor. Is there any way of telling if it's the
upper or lower sensor, without spending $150.00 for both?

 

I've had the same thing going on for a couple months now, same vehicle too.
It will stay off for a couple weeks and then back on again for a couple
days. I purchased an Actron OBD II scanner, read the codes, and found out
it is the upstream sensor. Rather difficult to get to, probably have to
pull the intake manifold. I'm waiting for cooler weather before I attempt
that. Maybe you know someone who has a scanner you could borrow, or pull in
to Autozone or Checker and get a free scan. Also, haven't had much luck in
finding a proper tool to remove/replace the device. The upstream sensor
sits in a hollow on top of the exhaust manifold junction where you can't get
a wrench on it so you need one of those special sockets with a cut-away to
clear the wiring. I can't find one long enough to accept the size of this
new Bosch O2 sensor.