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The NOx trap genuinely at the end of its useful life, no longer storing oxides of nitrogen. It is the conclusion of last resort, because sulphur poisoning, incomplete purge cycles and a faulty NOx sensor all produce the same efficiency code far more often and cost far less.

Ageing is thermal. The storage coating sinters at high temperature, permanently reducing surface area — so a trap that has been through repeated over-temperature events, or sat behind a misfiring engine, ages faster than its mileage suggests.

The evidence that separates it from a recoverable condition is a desulphation cycle that completes and changes nothing. Confirm the NOx sensors first, since a sensor reading incorrectly will condemn a healthy trap and is a fraction of the cost. Two sensors disagreeing about a trap that has just been successfully purged is the sequence that points at the trap rather than at anything cheaper.