My X5d also called for DEF at 900 miles. Dealer had me stop in and they found exactly what MINIz Guy's dealer found (100% in main tank, 66% in passive tank). The temperature was 35F outside.

They checked for leaks and didn't find any. So they gave me a bottle of DEF fluid to have at home and sent me on my way.

This dealer has sold quite a few diesels, and I'm convinced they took it seriously. In any case, no warnings for the last two days when the temps have gotten down to single digits - although my X5d is always parked indoors.

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2012 F10 550xi Carbon black/oyster
2011 E92 M3 SSII/black
2012 W166 ML63 Steel grey/porcelain
2007 E93 monaco blue/beige (retired)
2010 E70 35d Space grey/saddle brown (retired)