Cause
Ozone reduction catalyst coating worn out
The radiator's ozone conversion coating worn away with age. It is a thin catalytic layer on the leading face, and it is eroded by exactly what a radiator meets: grit, salt, insects and water at speed.
High-pressure washing is the accelerant worth naming. A lance held close strips the coating and flattens the fins in one pass, so a vehicle that has been detailed enthusiastically can lose it early. The module infers efficiency rather than measuring the coating, so other faults reaching that calculation produce the same code — check that airflow through the matrix is genuinely unobstructed first, since a blocked radiator reduces the same efficiency the coating is blamed for. There is no repair short of replacement, and the correct part is the coated one.
Typically a moderate job.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
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