Cause
Uncoated replacement radiator fitted
A plain radiator fitted where the vehicle came with a catalytically coated one. Some vehicles carry a coating on the radiator that converts ground-level ozone as air passes through — it is an emissions device rather than a cooling one, and the module monitors its efficiency.
A plain replacement cools perfectly and converts nothing, so the vehicle overheats never and stores an efficiency code permanently. That combination — a new radiator and a persistent efficiency fault — is the signature. The part looks identical, which is the whole difficulty: the coating is not visible and a catalogue listing by dimensions will offer the wrong one. Check the part number against the vehicle identification number, and expect the correct part to cost noticeably more.
Typically a moderate job.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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