Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove plug marked "Opentherm | Weather compensation" and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the boiler still fire with no demand?
Note:
Opentherm connections will be a link unless boiler connected
to an opentherm system. Test is the same whichever is fitted.