Cause
Air entering the engine downstream of the point where it is measured, so the engine burns more air than the module believes it received. This is the single most useful idea in intake diagnosis, and it is why an airflow sensor is the most over-replaced part on the vehicle: the code names the measurement, the fault is the leak, and the two are in different places.
The position of the leak decides which code appears. A leak that is large relative to total flow shows worst at idle and closes up under load, so lean and idle complaints dominate; one that only opens when a hose flexes or a gasket lifts under boost shows the reverse. A sudden high airflow reading with no corresponding pedal input is the throttle system seeing air it did not admit.
Look for it in the order things fail. Perished vacuum hoses and their tees, the crankcase ventilation circuit, the brake servo hose and its check valve, an intake manifold gasket, a throttle body gasket, and any pipe disturbed by recent work. A smoke test settles it in minutes; fuel trims tell you a leak exists but never where it is.
Typically a moderate job.
Codes where this is a likely explanation first, rather than merely a possible one.
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Air that enters after the airflow sensor is air the ECU never metered — the engine runs lean, trims climb, and half this site's most-visited codes trace back to it. A smoke machine...
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