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The linkage between an actuator and the flap it moves, broken so the flap stays put. The actuator works, its feedback may even report movement, and the flap does nothing — which is why the module reports a performance fault rather than a circuit one.

Intake manifold flaps are where this is commonest. The linkages are often plastic and live in a hot, oily environment, so they embrittle and snap at a pivot or lose a clip. The symptom is the flap's function simply absent — a flat spot at one end of the rev range for a manifold flap — with no code naming anything mechanical. Look at the linkage before testing the actuator: a broken rod or a detached ball joint is visible in seconds and is the likeliest finding on an older engine.