The valve should open fully at a coolant temperature of 20*C and close when it rises to 70°C.
To check the valve on a cold engine, remove the valve box and blow into it: air should pass. If not, then the valve is faulty.
To check the valve on a warm engine, start the engine and let it run, idling, pinch the hose between the valve and the manifold. The engine speed should not change. A change in the speed of the crankshaft indicates a jamming of the control plunger 2 (see photo) and the need to replace the valve.
To replace the valve, drain the coolant, disconnect the cooling system hoses and remove the valve.
When installing a new valve, replace the seal.