Check out my post here:
https://xoutpost.com/bmw-sav-forums/...st-system.html

and then check your car where the headers meet the first part of the exhaust system. It is said that if there is a leak there air gets sucked IN and gives that post O2 sensor false readings.

Have you checked for vac. leaks yet? You make a decent smoke tester with a cigar and manual pump for $6 from Harbor freight

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2004, X5, 3.0, 202k+