To check the valve on a cold engine, remove the valve box and blow into it: air should pass through. If not, the valve is faulty.
To check the valve on a warm engine, start the engine and let it run, while idling, pinch the hose between the valve and the manifold. The engine speed should not change. A change in the crankshaft speed indicates that the control plunger 2 is jammed (see photo) and that the valve needs to be replaced.
To replace the valve, drain the coolant, disconnect the cooling system hoses and remove the valve.
When installing a new valve, replace the sealing gasket.