Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Isolate incoming water supply to appliance
Remove seasonality valve from appliance

Observation:

Remove filter from the inlet of the seasonality valve.
Inspect the filter, it should be clear and unrestricted.
Is the filter clear and unrestricted?
Note:
If filter is clear do not refit seasonality valve at this stage.