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The spring returning a control valve to its default position, broken so the valve no longer returns when the actuator releases it. The default position is a safety design — a valve is arranged to fail toward whichever state is least harmful — and a broken spring removes that guarantee.

The fault is asymmetric, which is its diagnostic signature. The valve still moves in the direction the actuator drives it and fails to come back, so the module commands a change, sees the position achieved, then commands the return and sees nothing. A valve that moves one way correctly and not the other is a spring, not an actuator — an actuator fault degrades both directions.

Where the valve stops determines how serious it is. A recirculation or bypass valve left open can flood the intake with exhaust at idle and stall the engine; one left closed simply stops the function. Test it by hand with the actuator disconnected: it should snap back positively to its rest position, not drift or stay put. Carbon seizure produces the same symptom, so establish whether the valve is free before concluding the spring has broken.