As far as California is concerned, Ric's got it right -- it's
bureaucratic and political.  The California Bureau of Automotive Repair
(BAR) goes by a Bible that for other than mass produced models is error
prone.  We (me and the BAR) used to play the every-other-year smog game
with my gray market 911, bought in Europe. It always passed the sniffer
but had a distributor that wasn't on the list of approved items. The car
would fail visual inspection at the local smog guy and I'd go the 40
mile drive to the "referee station" (a private contractor that answers
to no one). They would issue a notice of failure. I would mail that to
Porsche America in Reno.  Porsche would return a form letter that my
distributor was an "authorized substitute."

Back to the BAR referee station with the Prosche letter. They would pass
the car. Two years later it was back through the same process. The prior
correspondence didn't count because the manuals hadn't been revised to
include my distributor. Etc.  Etc.  Etc.

Finally, I got smart and took the critter to a Prosche dealer for smog.
   California law let the dealer sign off on the distributor, OK the
car and pass it.  $ 105 bucks for a smog inspection at the dealer was
cheaper than $ 45 at the corner smog guy...figure.

These people look at the VUN.  For a given VIN there's a configuration
list of things they look for.  If I had taken my European 911 and
upgraded it with all California approved components it would have failed.

I have seen inspection options on the smog testing machines that provide
for two CATs but don't know what that might mean.  I fear that getting
the two banks into the ball park might be tougher than the average of
the two -- maybe an ECM averaging two O2 sensors???  Next time someone
goes for smog it might be a good deal to ask and post the answer here.

PJ