When the measured value is less than the nominal value - 0.2 bar

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2016 BMW 760Li, 2015 BMW 760Li, 2014 BMW 760Li, 2013 BMW 760Li, and 2012 BMW 760Li. However, it is still accessible from the selected car via links, so may be relevant.

The line cross-sections in the fuel feed are constricted or the fuel filter is plugged, or
The voltage supply to the fuel pump function is not OK: For example, as a result of high con resistance (corrosion) in the plug connection between the wiring harness and the fuel pump