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On engines using high pressure oil to actuate the fuel injectors, the pump generating that pressure, worn until it cannot reach or hold the commanded value. This is a separate circuit from the engine's lubrication system and runs at pressures an order of magnitude higher; confusing the two sends the diagnosis to the wrong pump entirely.

Its failure shows worst where demand is highest — cranking, cold starts and full load — because a worn pump can still satisfy a light demand. So the complaint is a long crank or a no-start when cold with an engine that runs acceptably once going, and the stored codes name injection pressure rather than the pump.

Distinguish it from a leak in the circuit, which presents identically. A pump that cannot build pressure and a circuit that cannot hold it both show as commanded pressure not achieved. The difference is in how the pressure behaves when the pump stops: a circuit that bleeds down rapidly has a leak, while one that holds points at the pump's ability to generate. Test that before ordering, because the pump is the expensive part and a failed seal or fitting is the commoner fault.