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Sulphur from fuel accumulating on a NOx adsorber's storage sites, displacing the oxides of nitrogen it exists to trap. Sulphur binds more strongly than NOx does, so the trap fills with the wrong thing and its capacity falls.

It is largely reversible, which is what distinguishes it from ageing. The module runs a periodic desulphation cycle — a deliberate high-temperature, rich excursion that drives the sulphur off — so a trap losing efficiency is expected to recover after one. A trap that does not recover after desulphation is genuinely degraded, and that distinction decides whether the part needs replacing.

High-sulphur fuel accelerates it, which matters in markets where fuel quality varies. Check whether desulphation cycles are completing before condemning the adsorber: cycles that are being aborted — by short journeys, by a temperature fault, or by another inhibiting code — leave the sulphur in place, and the fault is then the aborting rather than the trap.