There is an active and a passive urea tank. Fluid is transferred from the passive tank to the active tank where urea from the active tank is then injected into the exhaust stream. By design, the active tank is kept full by this transfer process. As long as there is fluid in the passive tank, that is. The passive tank is large and is constantly being consumed by the transfer process. The fill port for the active tank is on the right side of the vehicle and passive tank fill port is on the left side.

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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