Cause
Disconnected crankcase ventilation hose
A crankcase ventilation hose off its stub or split open, which is a large unmetered air leak straight into the intake. The whole circuit sits downstream of the airflow sensor, so everything it admits is air the engine never measured — and this hose is a bigger bore than most vacuum lines, so the leak is substantial.
The result is a lean condition that is worst at idle, together with an idle that hunts or will not settle, and fuel trims driven to their positive limit. Modern systems detect the disconnection directly and name it, which is more helpful than the generic lean code it would otherwise produce.
It also stops the crankcase being ventilated, so pressure builds and finds the weakest seal — meaning an oil leak appears as a second symptom from the same cause. Check the hose's whole run rather than its ends: these harden with heat and split on the underside where the crack closes when the engine is off. Suspect it after any work in the area, since it is routinely removed to reach other components and easy to leave unclipped.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (3)
Codes where this is a likely explanation first, rather than merely a possible one.
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