BMW Repair: 740IL 1998, smog test, oxygen sensors

Question
I'm having a had time passing smog test with my 740. I've replace both cat-converters last year and it did pass but barely, this year the nox is twice the limit in California..any suggestions on what to do.

thank you.

Answer
Nelson, NOx is a byproduct of efficient high heat combustion. It should be reduced if not eliminated by the Cats. Is the motor running hot? or is the engine running lean? Check for air leaks in the intake. Make sure the cooling system is functioning well. You can try swapping in Oxygen sensors. Especially if you haven't replaced them at 100K miles. If you purchased aftermarket Cats then try swapping a buddy with a factory cat exhaust. I think you can get a pretest or a 5-gas printout without running the full smog test.
-Evan