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A filter carrying more soot than it should, but soot that will still burn — the recoverable state, as distinct from a filter permanently restricted by ash. Keeping the two apart matters because they look identical to the driver and cost very differently to put right: this one is usually solved by allowing a regeneration to complete, and the other is solved by a new filter.

It is a consequence rather than a fault in itself. Soot loads faster than it burns when journeys are too short for the exhaust to reach regeneration temperature, when regenerations are begun and repeatedly interrupted, or when something upstream is producing excess soot — worn injectors, a leaking turbocharger seal, a stuck exhaust gas recirculation valve. So a heavily loaded filter is a question, not an answer.

Its effects reach well beyond the filter. A high load raises the temperature the system will tolerate during regeneration and pushes exhaust temperatures up across the aftertreatment, which is why this cause appears on over-temperature codes and on catalyst codes rather than only on filter codes. Check the load percentage and the count of incomplete regenerations together — a high load with many aborted attempts names the fault far better than either figure alone.