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The module governing the additive system, faulty so dosing is not commanded or not tracked. It is a small dedicated controller on the vehicles that carry one, and its faults surface as invalid data reported by the engine module rather than as anything the driver perceives.

The consequence is the same as an empty tank: no additive reaches the fuel, the filter's regeneration temperature stays high, regenerations are aborted, and the filter loads. So the visible fault is a particulate filter problem and the cause is a module two steps upstream.

Rule out the system before the module. An empty tank, a counter that was never reset, a blocked dosing line and a failed pump all produce module-level faults — and the module refusing to dose is usually correct behaviour rather than a failure. Check the level, the counter and the pump's delivery first, since all three are cheaper and commoner than the controller.