Yes! An exhaust leak [cracked exhaust manifold, bad gasket where manifold connects to exhaust or exhaust header to block] allowing outside air to be sucked into cat converter seems to be a very common source on the board when all is said and done for high NOX. Unfortunately it is the last thing people find after replacing O2 sensor, cat converter etc. I' afraid my car has developed a leak based on the burble sound of exhaust in the manifold area, my NOX was barely passing last time. So sound is one tip.
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1988 245A. Past: 1979 245A; 1987 244 M47; 1971 164E

Do you know about plugging the vacuum line coming out of the diaphragm at the ECU? (right front fender) Not getting a signal, the spark won't advance so much and this helps with NOX. The car will still run well enough without the extra advance.

Steve

High NOx is due to high combustion temperatures. So looking for an air leak is a good idea. Check the O2 sensor output to see if you can verify a lean condition.

If running lean, how is the engine smoothness at idle?
Check the plugs, if one is different it would indicate a manifold gasket leak.
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84 242Ti IPD bars&springs, 89 745 16v M46 IPD bars, 89 744 16v M46 IPD bars, 93 945 Turbo AW71, 91 245SE AW70 IPD bars, 93 245 CLassic M47

Worst case is that you need a new cat.

Yes, I know that you replaced the cat last year. But some conditions of the car will destroy the cat in a week! One professional mechanic who used to contribute to this forum called the condition, "cat for a day". According to him, if the car isn't set up right (i.e., f/a mixture), the cat is quickly destroyed. If you don't find some other solution, putting on a new cat, again, is your last resort.

Regards,