Bad NOx sensors or urea dosing module. Just guesses but both are the subject of a 10 year or 120k miles extended warranty for certain diesel X5s. Worth getting it checked out but do your research and get a copy of the BMW NA letter first. Unless your dealer and service advisor are good and will put in the time to get this done for you. Left to their own devices they'll just charge you for the repair and parts.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim