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

WARNING: This page is about a different car, the 2011 BMW 750xi, 2011 BMW 750Lxi, 2010 BMW 750xi, and 2010 BMW 750Lxi. 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