Originally Posted by e39_touring

Google BMW eu2 tune for m54. It's free! You just need an obd2/inpa cable (about $15 on ebay) and inpa/winkfp which can be downloaded on various forums.

This will eliminate the post cat sensors from the tune and will allow your x to operate properly without therm. A lot of folks that remove their cats and delete the secondary o2 sensors complain of exhaust stink. The engine runs pig rich if you remove the secondary o2 sensors without properly tuning them out. The eu2 tune is the factory tune for BMWs spec'd without cats in other markets.