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A leak in the vacuum lines operating a control actuator, so the actuator receives less vacuum than commanded and moves short of its position. It is the actuator's supply that has failed, not the actuator — which is why the codes name the valve's performance while the valve itself is in good order.

Vacuum-operated actuators are still common on emissions hardware, particularly recirculation and bypass valves, and their lines are small-bore rubber routed through the hottest part of the engine bay. They harden, crack at the ends where they push onto stubs, and split on the underside where nobody looks.

The symptom is a valve that partially works, which is the confusing part. A leak reduces available vacuum rather than removing it, so the valve moves but not fully, and the module reports a position or flow error rather than a failure. Test with a hand pump at the actuator — it should hold vacuum indefinitely and move fully. If it does, the leak is in the supply, the reservoir or the control solenoid; if it does not hold, the actuator's own diaphragm has failed, which is a different repair and a common finding on the same test.