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The small pump metering catalyst additive into the fuel tank, faulty so the dose is not delivered. It moves very small quantities at long intervals — a measured amount at each refuelling rather than continuously — which is what makes its failure so quiet.

Nothing is noticeable until the filter starts loading. Without the additive the filter needs a higher regeneration temperature, so regenerations take longer, are aborted more often, and the codes that eventually appear name the filter. The additive system is upstream of a fault that presents as a filter problem, which is the connection worth holding.

Its failures are electrical or blocked rather than worn. The additive is corrosive to some materials and crystallises in a line left standing, so a blocked outlet is as likely as a failed pump. Command the pump and confirm delivery rather than confirming it runs — a pump that operates audibly while nothing reaches the tank is the commoner finding.