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The shaft of a valve bound in its bore by carbon on the hot side or corrosion on the cool side, so the valve cannot move freely. It is the mechanism behind most valve performance codes in the exhaust and emissions systems, and it names the shaft rather than the valve face deliberately — the seat can be clean while the spindle is seized.

Binding rather than seizure is the common state and the confusing one. A stiff spindle lets the actuator move the valve slowly, or partway, or with an overshoot as it breaks free — so the module reports a position not reached in time rather than a valve that failed. A fault that appears only when cold, easing as things expand, is binding.

Check the effort by hand with the actuator disconnected. A healthy spindle turns smoothly through its full travel with light pressure. Watch the actuator's current draw if the tooling allows: a motor working unusually hard while still reaching position is the early warning that arrives before any code. Cleaning is a repair with a lifespan unless what deposits the carbon changes.