Pompe à huile usée

The pump circulating engine oil, worn to the point that it cannot maintain pressure — and on an engine with a variable-displacement pump, one whose control has failed as well. It is the cause most feared and least often found, because almost everything that reads as low oil pressure is something other than the pump.

Take the reading seriously and then verify it. A genuine loss of pressure destroys an engine quickly, so it is never something to drive on. But the sensor, its wiring, a blocked pickup strainer, a stuck relief valve, worn bearings increasing clearance, or oil thinned by fuel dilution all reduce indicated pressure with a healthy pump. Confirm with a mechanical gauge at the sender port before any conclusion — it is the one measurement that cannot be argued with.

Wear is usually a symptom rather than a cause. A pump runs in the oil it pumps, so grit from a neglected engine or debris from a previous failure wears the rotors, and pressure falls hottest and at idle first — the conditions where clearance matters most. A pump replaced without cleaning the sump and finding what wore it will wear again, and on a variable-displacement pump check the control solenoid and its screen before condemning the pump body, since a stuck control gives low pressure with no wear at all.