Wow. I would be really PO'd if I failed with a new cat. Bummer.
We had high NOX readings on the 525 we used to own.
Here is what Bill R explained about that.
Originally Posted by Bill R.
Our NOX problem was really bad. Allowable Max is 767/706 Our car 884/1124. I got it down over 50% and passed so there is always hope. Here is the whole thread.
http://www.bimmer.info/forum/showthread.php?t=12713
This other current thread has some good suggestions:
http://www.bimmer.info/forum/showthread.php?t=26432
I am not sure anyone ever knows why a car fails. You just throw every trick in the book at it and try again.