Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Drain boiler primary circuit

Observation:

Remove and inspect CH filter for blockages / restrictions.
Information:
Some appliances may not be fitted with a CH filter. Unscrew top nut of flow manifold and push air separator back slightly, if filter is fitted it will be visible, removing filter to inspect may require condensate trap or air separator to be removed.