Hello.  Here is my situation.  Opinions and advice solicited:

- I have a 2001 Z28 with an LS1 in it.
- Very recently the SES light came on.
- Car seems to run fine, no noticeable change.
- Had it scanned for codes at a parts store.
- Code thrown is P0430, Catalyst efficiency below threshold (bank 2).

I'll throw these in also for completeness but separate them because
they may run the risk of distracting from the real issue:

- Approximately a week or so prior to code coming on, I put the
proscribed amount of Sta-bil fuel stabilizer in the tank.  I put it in
a quarter full tank then did a complete fill up to mix it as best as I
could.  I sometimes do not drive the Z28 in the winter months much at
all and I was concerned about the fuel getting too old over the
winter.  This is the 1st time I have used this product on this vehicle
but have used it in the past on other things.  I have put a fresh 3/4
of a tank of fuel in since and have not added more Sta-bil.  I have
been hoping the problem would go away but hasn't.
- Many months ago I needed to have an air pump check valve replaced
and I replaced both sides.  No problems after doing so (until now).
- Ever since I have owned the car I have always thought the exhaust of
this car smelled a bit like, strangely enough, like the smell of
freshly fired ammunition.  Whereas I haven't really thought that of
any other exhaust so much.  Other exhaust has always hinted more to me
of burnt up gasoline (big surprise there).

I believe that my exhaust is configured in this way: exhaust for each
two  banks of four cylinders is an exhaust manifold which leads to a
common catalytic converter with oxygen sensors for each bank before
hitting the catalytic converter and a sensor in common after the
converter.

My understanding of how this code might be generated is that the
computer will evaluate the catalyst efficiency by comparing the oxygen
sensor readings before the cat and after it.  My "logic" therefore
dictates that a likely problem is a failing oxygen sensor in the
exhaust stream coming off bank 2.  If the catalyst itself was actually
not working, wouldn't codes be thrown for both banks?  If there were a
problem with the sensor for bank 1, wouldn't the code be thrown for
that bank?  If the common oxygen sensor after the cat were bad,
wouldn't it think both banks were bad?  Must the answer be
fouled/failing sensor in bank 2 exhaust stream?  Am I completely
missing something?  Thoughts?  I am tempted to just go and buy a new
sensor (or set of them) and install but before I do I would value the
advice of some of you so I don't waste any more money or time than I
have to.