look on Wikipedia about the QR25DE motor and catalytic problems.

BTW,the UPstream O2 sensor determines the fuel mix,and the rear downstrteam
O2S checks the cat to see if it's there and working.

The usual problem with cats is that they clog up,become an obstruction.

An aftermarket cat should be fine.  I believe there are federal
standards with which any cat sold in this countrty must comply.  Check
with the auto parts store and you should be able to find a good one
that will fit and be a lot cheaper than the dealer.  The universal
ones often have the correct size flanges on either end and a sliding
tube in between that permits installation in a variety of applications
that use that flange size.  Usually cat failure results from the
extremely high heat generated by the cat and results in disintegration
of the inards of the cat which could, though not too likely, cause a
blockage in the cat.  Be prepared to hacksaw out the bolts holding the
old one.  Guaranteed they are thoroughly fried and rusted.  Be sure to
use liberal amounts of antiseize compound on the new bolts and all
mating surfaces.  Even so, the high heat will tend to fry the bolts.
It's worth turning them and applying new anti-seize compound every so
often when you happen to be under the car.  It will facilitate future
exhaust repairs.  Good luck.  Al

the spacer should be in the DOWNSTREAM O2 sensor position.AFTER the cat.

do a Google search for "quest aftermarket catalytic converter".

http://www.autopartswarehouse.com/search/?Ne=28&N=9190&Nr=AND(universal:1)&
V=11384+1684+4294967022&Vh=2004%20Nissan%20Quest

Looks like there might be some hope after all...  I was checking out this
website that sells Nissan brand parts at a discount:

http://www.worldpartsexpress.com/partlocator/index.cfm?
action=getLocator&siteid=214078&chapter=APF602&appSectionid=7&groupid=
60222&make=25&model=Quest&year=2004&catalogid=1

They have the Nissan brand cat for the 04 quest for $420, but it also
says that it's the same part used on these cars:

95-98 Nissan 200sx
95-06 Altima
95-06 Maxima
95-06 Sentra

That seems a little hard to believe, but I'm going to try and see if
front right cat part for the 04 Altima, Maxima, and Sentra are the same
part # at other places.  If so I can probably find an aftermarket cat for
one of those that should fit on the Quest as well.

Also - does anyone know if the Altima cat problem (the one where the
chemicals get sucked into the engine and destroy it) affects other models
as well?  Wondering about my Quest in particular.  Seems like if it has
the same engine & cat as the problem models from 01 and 02 I might be in
for some real trouble.

Thanks!