I am the owner of an '03 VW GTI 1.8T with just over 94k miles.  The
check
engine light has been popping up every now and then over the past month
or so.  I am hoping someone here might be able to help me out.

First time the CEL came on, I took it to my local mechanic, who pulled
the code and reset it for me, just to see if it would come back.  The
CEL came back a couple hundred miles later.  Second time, I took it to
AutoZone to have the code read myself - they pulled P0421.

OBDII code P0421:  "The PCM has determined that the warm up catalyst
efficiency for bank 1 is below threshold for the current engine
operating conditions."

I had AutoZone reset the code as well.  Now, the CEL came on again this
morning after another few hundred miles of driving with it off (and a
few cold morning starts here in NJ).

I've done a bit of research and I think I have the problem narrowed
down to two possibilities:

1.) The second O2 sensor (downstream of the cat) is dirty or going bad,
the computer is interpreting this as a low cat efficiency reading, and
the O2 sensor needs to be cleaned or replaced.
2.) The catalytic converter really is going bad and needs to be
replaced.

I was leaning towards a dirty/bad O2 sensor, because the CEL takes
awhile to light up after it is reset.  My thinking was that if the cat
really was running at low efficiency, the CEL would come on much more
quickly.  A dirty or bad O2 sensor might throw a funky voltage at the
computer only every once in awhile.

Also, the car passed inspection two weeks ago.

Am I on the right track here?  How do I fix this?  I'm in the midst of
trying to sell the car and I don't want to have any issues with the CEL
once the car is out of my hands.