تدهور حشوة ممتز الهيدروكربونات

The hydrocarbon trap's substrate degraded, so it no longer stores the unburnt fuel produced before the catalyst lights off. It exists to cover the first seconds of a cold start, when the catalyst is too cold to convert anything and most of a journey's hydrocarbon emissions are produced — the trap holds them and releases them once the catalyst is hot.

Its degradation is thermal and permanent, so a trap that has been through repeated over-temperature events ages faster than its mileage suggests. The fault produces no driveability symptom at all, which is why it is found at a test or by the monitor rather than by anybody noticing.

Rule out the cause of any over-temperature before replacing it, since the same misfire or rich condition that sintered the substrate will sinter the replacement. Check that the monitor's preconditions were actually met, too — these monitors need a genuine cold start after a long soak, so a code from a partial cycle is worth confirming across a proper one.