Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Link out the opentherm terminals as shown in the image
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the boiler now recognise and respond to a central heating demand?
Note:
If the boiler now responds to the central heating demand do not remove the link at this stage.