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The additive tank empty on a vehicle that doses a catalyst additive into the fuel. Some diesels use an additive to lower the temperature at which the particulate filter regenerates, held in its own small tank and metered in at each refuelling — and it is a consumable that many owners do not know exists.

An empty tank means regeneration needs a higher temperature than the vehicle can readily reach, so regenerations become harder to complete, the filter loads, and the codes that follow name the filter rather than the additive. A filter loading on a vehicle whose additive tank is empty is a consequence.

It is not a service item on most schedules, being refilled at a long interval, so it is missed. Check the additive level and the dosing counter together — and note that refilling alone is often not enough, since the counter that tracks remaining additive has to be reset for the module to believe the tank is full.