Cause
Aged or contaminated SCR catalyst
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.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (5)
Codes where this is a likely explanation first, rather than merely a possible one.
- P24FE SCR Catalyst Incorrect Likely for this code High
- P20EE SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 1 High
- P20F5 Reductant Consumption Too High High
- P214D SCR NOx Catalyst Outlet Temperature Too High During Particulate Filter Regeneration High
- P249E Closed Loop Reductant Injection Control At Limit - Flow Too High Moderate
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