Cause
Loose intake ducting clamp
A clamp on the intake ducting loose, so the joint admits unmetered air or releases boost. It is the simplest fault in the intake and one of the commonest, and it is almost always the result of recent work rather than of anything failing.
Which symptom appears depends on where the joint is. Between the airflow sensor and the throttle it is a vacuum leak — air the engine never measured, so the mixture goes lean and the idle hunts. On the pressurised side it is a boost leak, audible under acceleration and invisible at idle.
A clamp does not need to be off to leak. One that has relaxed, or was tightened onto a duct that had slipped partly off its stub, will hold at rest and release under pressure or engine movement. Check every clamp by hand rather than by eye, and pressure-test the charge side rather than looking for it running — a joint that only opens under boost passes every inspection on a stationary engine.
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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