This has been a really informative thread. I have a US spec model in Germany. I just got my "no start in 992 miles" warning, at about the 18,5000 km mark. So I stopped into the local Esso station, and picked up a five liter jug of Ad Blue, with spout. I only filled the active tank, but now that I know the passive tank exists, I know where the remainder is going. The dealer noted the level check as OK at the first annual service, but it may be they too were unaware of the two tank requirement. Good news is the stuff was only 10 Euro for 5 liters.

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2009 X35d Space Grey