Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the wires (or link) at terminals marked "OPENTHERM"
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?
Notes:
1. Opentherm connections will be a link unless boiler connected
to an Opentherm system. Test is the same whichever is fitted.
2. Wires for weather compensation must stay connected.