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

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2012 BMW 328xi, 2012 BMW 328i, 2011 BMW 328xi, 2011 BMW 328i, and 2010 BMW 328xi. However, it is still accessible from the selected car via links, so may be relevant.

Line cross-sections in fuel feed are constricted or fuel filter is clogged, or
Fuel pump voltage supply is not O.K.: e.g. as a result of high contact resistance (corrosion) in plug connection between wiring harness and fuel pump.