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A hydraulic accumulator no longer holding its charge, so the circuit loses its store of pressurised fluid. The accumulator exists to supply pressure faster than the pump can build it — for a shift, an actuation, or the moment after a start-stop restart — and one that will not hold turns an instant response into a wait.

Its failure has a distinctive signature: the pump cycles far more often than it should. With nothing storing pressure, the pump refills continuously, and a cycling-period code is exactly that observation. A pump running frequently is the symptom to recognise, and it points at the accumulator rather than at the pump that appears to be misbehaving.

Most accumulators are gas-charged behind a diaphragm or piston, and the failure is the gas leaking away or the diaphragm rupturing. The unit then behaves as a plain section of pipe — no fault visible, nothing leaking externally, and every pressure test passing while the vehicle is running. Watch how quickly pressure decays with the pump stopped: a healthy accumulator holds it for a long time, and a failed one drops almost immediately. That test distinguishes it from a leak elsewhere, which decays more gradually.