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The selective catalytic reduction catalyst no longer converting enough oxides of nitrogen, either through normal ageing or because something has poisoned it. It is the conclusion a module reaches after ruling out the reductant system, and it should be the conclusion a diagnosis reaches last too — almost everything that reduces conversion sits upstream of the catalyst and is cheaper to fix.

Contamination has identifiable sources. Sulphur from poor fuel, engine oil carried through by worn turbocharger seals or a failing crankcase circuit, silicone from the wrong sealant used on an exhaust joint, and coolant from an internal leak all coat or chemically poison the active surface. Thermal damage is the other route — repeated or uncontrolled regenerations sinter the coating, and that is permanent.

The evidence that separates a genuinely dead catalyst from a starved one is the reductant dosing figure. A catalyst that is aged converts poorly while dosing normally; a system dosing far more than expected and still failing is usually delivering reductant that is diluted, crystallised or not reaching the stream at all. High consumption alongside poor conversion argues for the delivery system, not the catalyst, and that is the more common finding.