Cause
Faulty intake air oxygen sensor
The oxygen sensor in the intake rather than the exhaust. Its job is to measure the oxygen content of the charge entering the engine, which matters on engines using heavy exhaust gas recirculation — recirculated gas displaces oxygen, and this sensor measures how much.
It lets the module control recirculation directly rather than by inference, so a fault costs the accuracy of the whole strategy: recirculation rates are estimated instead of measured, and the module reports correlation faults against its model. It is easily confused with an exhaust sensor in the codes, and it is in a completely different place. Its position exposes it to oil mist and recirculated soot, so a coated element responding slowly is commoner than an electrical failure — compare its response speed rather than its value.
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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