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

WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2005 BMW 325i, 2005 BMW 325Ci, 2004 BMW 325i, 2004 BMW 325Ci, and 2003 BMW 325i. 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 cont resistance (corrosion) in the plug connection between the wiring harness and the fuel pump.