Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Remove all external control wiring connections and make safe
Link out external controls as indicated by the red wire link in the photo
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Cancel purge cycle.
Does the appliance now recognise and respond to the
central heating demand?
Note:
If the appliance still does not recognise demand, leave the link in place until diagnosis is complete.