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Carbon deposits on the wastegate valve and its seat, so it no longer seats fully or moves freely. The wastegate controls boost by bypassing exhaust around the turbine, and it lives in the exhaust stream, so fouling is a matter of time rather than a defect.

Which way it fouls determines the code. Held partly open by deposits, exhaust bypasses the turbine continuously and the engine underboosts. Seized closed, nothing bypasses and boost runs away, which is the more damaging direction. Both produce a wastegate performance code with an actuator that is working correctly.

Distinguish it from an actuator fault by feel. Disconnect the actuator and move the wastegate arm by hand: it should move through its full travel smoothly and return positively. A gritty, stiff or partial travel is fouling, and an actuator that cannot move a free valve is the actuator's problem. Check for exhaust leaks at the same time — a leak upstream of the turbine has the same effect on boost as a wastegate stuck open.