Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove DHW micro switch head

Observation:

With the hot tap turned off the spindle should be retracted.
Test 1:
Turn hot tap ON.
Does the spindle extend?
Test 2:
Turn hot tap OFF.
Does spindle retract?
Expected movement 4mm
Note:
Repeat this test several times to confirm the valve is not intermittently sticking.