Tired of problems with hesitation and roughness below 5K rpm and BMW's lack of help concerning it? I have been living with the problem for the last 30K miles. I felt it should be a warranty problem and should be fixed. I went for a ride today and could not take it any more. I stopped at Napa and bought a new O2 sensor. Part number 15730. It was 64 bucks plus tax. It came with bare wires(no plug). I spliced her in and went for a ride. She rides so much better now. I wish I would have done this a long time ago. So, If your thinking about it, Just do it! Thanks to Ken, Grif, and others for the info on this.