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Pressure on the low pressure side of the fuel system below what the high pressure pump needs to work with. It is a supply fault reported by the high pressure system, which is what makes it worth naming: the codes name rail pressure or injection, and the shortfall is upstream of all of it.

A high pressure pump cannot make up for a poor feed. Starved, it cavitates, fails to reach commanded rail pressure, and wears rapidly — so a lift-side problem left alone becomes an expensive one. Rail pressure that falls short under load with the pump at maximum duty is the pattern to recognise, and it is the module telling you it is asking for everything and not getting it.

The causes run in a clear order of likelihood: a blocked filter, a restricted or collapsed line, a weak transfer pump, a strainer blocked in the tank, or air drawn in on the suction side. Measure supply pressure at the high pressure pump's inlet — that single reading separates a lift-side problem from a high-side one and decides which half of the system to work on.