Cause
Faulty humidity sensor
The sensor measuring humidity in the intake air, faulty so the module cannot correct for water content. Humid air contains less oxygen per unit volume, and it changes how readily the mixture ignites — so the module uses it to trim fuelling, timing and, on a diesel, recirculation rates and NOx modelling.
A wrong reading produces a small error everywhere rather than one obvious fault. Fuel trims drift, emissions modelling is off, and the codes name correlation or rationality rather than a failure. It is frequently combined with the intake air temperature sensor in one housing, so a single failure produces two unrelated-looking codes.
Its position exposes it. Mounted in the intake, it meets oil mist from the crankcase circuit and dust past a poor filter seal, and a coated element responds slowly rather than incorrectly. Compare its reading against ambient conditions after an overnight soak — a sensor disagreeing with the actual weather has an offset fault that no circuit test will reveal.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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