Emissions Failing 91 525i

[junior525i]

Please help, my car is getting a reading of 767 on the NOx, eveything else is passing but not the NOx, the smog guy said i have to get under 767 to pass, it might be my catolitic convertor but my Cat and my complete exhaust looks brand new. If the cat isnt genuine BMW, will it fail with aftermarket Cat?

also i feel very little low end power and my average MPG is around 17-19mpg's


PLEASE HELP

[Tiger]

Did you change your oil recently? If not, do it.

[junior525i]

yeah i did change it, also i have a stumble that i sometimes get when i start to accelerate.