A6804909536
SCR Assy. - 2 box · Detroit Diesel
SCR catalyst and combined SCR/particulate filter unit: urea dosing, part-number assignment and replacement checks
Selective catalytic reduction removes nitrogen oxides from the exhaust by injecting an aqueous urea solution (DEF) into the hot exhaust stream from a dosing module; the solution decomposes into ammonia, which reacts with the nitrogen oxides on the SCR catalyst to form nitrogen and water. On many newer engines, a combined one-box unit houses the particulate filter and the SCR catalyst together, reducing packaging space and heat loss. Temperature sensors and a downstream nitrogen oxide sensor monitor conversion efficiency.
The part number is tied to the substrate volume and the zeolite or vanadium coating of the SCR catalyst, to the geometry of the mixer ahead of the stage, and to the dosing strategy tuned for the engine family. On combined one-box units, matching with the integrated particulate filter adds a further variable, so part numbers differ between otherwise similar engine families. The assignment of part numbers on this page comes from official certification documentation issued by the US Environmental Protection Agency.
Crystalline deposits from incompletely hydrolysed urea solution in the mixer or at the dosing nozzle are a common failure pattern that makes injection uneven. A blocked or leaking dosing nozzle, a clogged DEF filter, or a faulty nitrogen oxide sensor trigger dosing faults and power reduction up to a start inhibit. At the SCR catalyst itself, thermal overload or overdosing leads to declining conversion and increased ammonia slip.
Replacement is governed by the part number, since substrate volume and coating can differ between engine families of similar size, and combined units also involve the integrated particulate filter stage. Checks should cover the sealing faces of the flanges, the condition of the dosing nozzle and its supply line, and the sensor connectors. After replacement, the control software typically requires a learning drive or an adjustment of the dosing strategy. Pricing and availability are not part of this page.
Excludes price and availability — those come from the supplier.
