NOX are oxides of nitrogen, the byproduct of burning hydrocarbons under high temperatures. This causes smog when it reacts with sunshine. The way that they overcame this was through the use of an EGR (Exhaust Gas Recirculation) Valve. This simply takes exaust and recirculates it back into the intake which cools the combustion temperature. This is the only device that they have to combat NOXs, the Cat does nothing against NOXs. It may have been possible that the EGR valve that the mechanic put on was no good and possibly stuck open (ours are electronic stepper motors, 3 valves, as opposed to vacume operated like most vehicles were). Two basic things you can do, lower your engines operating temperature and risk failing the hydrocarbons (by running a lower temp thermostat) or by trying another EGR. My first step would be to go with the EGR route.
1992 Bonneville SSEi
White, grey Leather interior.
Tinted windows and blackouts on headlights.
Custom high flow exhaust, Everything replaceable - DONE
300 000 kms original engine, tranny 2 rebuilds.
Playboy Bunnies Embossed in Tint. Oh Yah!
Ever Seen a Polar Bear in a Snow Storm?