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The pump that moves fuel from the tank to the engine's high pressure pump, called the lift or feed pump, and on many systems it is the second pump in the vehicle rather than the one people picture. Codes naming fuel pump B are usually about this device, so the designator is worth resolving before any part is ordered — pump A and pump B sit in different places and fail in different ways.

Its failure mode is loss of flow long before loss of function. A transfer pump rarely stops outright; it weakens, and the high pressure pump downstream starves under exactly the conditions that need fuel most — full load, high speed, a climb with a low tank. So the complaint is a power loss under load with nothing wrong at idle, which sends people to the turbocharger and the filter first.

Two checks separate it from what it is confused with. Measure delivery volume over a timed period rather than pressure alone, since a weak pump can still hold static pressure against a closed system. And check its supply voltage at the pump under load — a corroded connector or a tired relay drops enough volts to slow the motor, and the pump is then reported faulty when the wiring is at fault.