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A wastegate jammed at one end of its travel, so boost control is lost in one direction. Naming both directions together is deliberate — the mechanism and the causes are identical, and only where it stopped decides which symptom the driver reports.

Stuck open, exhaust permanently bypasses the turbine and the engine underboosts. The complaint is a general lack of power with a turbocharger that is mechanically fine. Stuck closed is the dangerous direction: nothing limits the turbine, boost climbs past its target, and the module will cut fuelling or power to protect the engine — so the complaint is a sudden and repeatable loss of power under hard acceleration rather than a constant one.

A vacuum or electric actuator that tests correctly points at the valve. Disconnect it and check the arm moves fully and freely by hand; carbon on the seat, corrosion on the shaft, or a bent linkage are what hold it. On a variable-geometry turbocharger the equivalent is the vane mechanism, which seizes the same way and produces the same pair of symptoms.