دائرة مفتوحة في أسلاك المضخة

The circuit feeding or controlling a pump is broken — a severed wire, a connector that has come apart, a corroded terminal that has finally stopped making, or a blown fuse.

The reason this cause deserves attention rather than a shrug is that a pump with no supply is indistinguishable from a failed pump. Both are silent, both deliver nothing, and both produce a code that names the pump. So the expensive part gets replaced to fix a broken wire, and where the pump is buried — in a tank, inside a transmission, under a battery pack — the labour costs more than the part. Checking that the pump is actually being fed is a few minutes' work and it comes first.

Pump wiring fails more than most for physical reasons. Pumps live in the places harnesses have the worst time: in wheel arches and under floors where salt and stones reach them, on or inside units that vibrate continuously, and often at the end of a long run where a small resistance has the most effect. Many are fed through a relay and a dedicated fuse, which adds two more joints that can fail and two more things that can be checked without tools.

Measure at the pump, not at the fuse box. A supply that is present at one end and absent at the other is exactly what an open circuit is, and a reading taken at the fuse proves only that the fuse is intact. Where the fault is intermittent, moving the harness while the circuit is watched will usually find the inch of wire responsible — which is worth far more than knowing the circuit is faulty.