Failed smog check

[lilboi]

My uncle recently gave me his 325is. I needed to swap the policy and ownership. In order for me to do that, i needed a smog check. I went to get a smog/emissions check and i failed! It read 1300 at 15mph in the NOX reading . That is almost twice the passing limit (about 700). Why could be causing this problem. The O2 sensor has been replaced. The only thing i can think of is the cat. If it is the cat, can i go to the muffler shop and tell them to weld on a universal cat? one more question will high flow cats pass smog?

[MrM3]

it could possibly be due to carbon deposits in the injectors, or crap in your intake manifold. knock on your cat. and if it doesnt rattle or sound like there's loose pieces in there it should be ok

[lilboi]

Well i just changed my cat yesterday. It cost me $211 for cat and installation. Hopefully now i'll pass smog. Wish me luck.

[lilboi]

Finally past smog with flying colors. The cat was suppose to be changed every 6 years, my car is approximately 16 years old. Its been overdue for a long time. My Nox reading at 15 mph decreased by 1/2 (700) and my reading at 25 changed severely, i only put out 35 (max 700, avg 120).