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

WARNING: This page is about a different car, the 2013 BMW X5 M, 2012 BMW X5 M, 2011 BMW X5 M, and 2010 BMW X5 M. 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 contact resistance (corrosion) in the plug connection between the wiring harness a the fuel pump.