I'm in NJ and just went through this. I ended up replacing a lot of parts but in the end it was the cat, twice. The previous summer when I was replacing the exhaust, I noticed the the catalyst in the converter was broken loose and disintegrating. I got an after market cat from FCP, but at the time the manufacturer did not differentiate between Bosch and rex regina FI systems, just egr or not. Well, after 5 tries and many parts, Nick at fcpgroton was able to find a manufacturer who makes a cat for b230 engines with the rex regina. With new parts, filters, sensors, etc, I was able to get my Nox down from 3200 to 1600, but never passing. With the new, correct cat, I was able to pass with no problems. It even helped the HC, CO, which were at the limits.

Good Luck,

Tom