Cause
Degraded reduction catalyst
The reduction catalyst no longer converting oxides of nitrogen at the required rate. It is the conclusion the module reaches after ruling out the reductant system, and it should be the last conclusion a diagnosis reaches too, because nearly everything that lowers conversion sits upstream and is cheaper.
Ageing is thermal and poisoning is chemical. Repeated or uncontrolled regenerations sinter the coating permanently; sulphur, oil carried through by worn turbocharger seals, and silicone from the wrong exhaust sealant poison it. Both are irreversible, which is what separates this from a dosing fault.
The dosing figure is the evidence that distinguishes them. A genuinely degraded catalyst converts poorly while dosing normally; a system dosing far more than expected and still failing is delivering reductant that is diluted, crystallised, or not reaching the stream. High consumption with poor conversion argues for the delivery system, and that is the commoner finding by a wide margin.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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