A method for modifying a NOx adsorber (23) regeneration triggering variable. Engine (20) operating conditions are monitored until the regeneration triggering variable is met. The adsorber (23) is regenerated and the adsorbtion efficiency of the adsorber is subsequently determined. The regeneration triggering variable is modified to correspond with the decline in adsorber efficiency. The adsorber (23) efficiency may be determined using an empirically predetermined set of values or by using a pair of oxygen sensors (27, 28) to determine the oxygen response delay across the sensors (27, 28).
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