Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove connector BUS from the PCB
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

With the BUS external controls disconnected does the boiler respond to a CH demand?
(Box appears around the radiator symbol on the display)
Note.
Ensure the wire link fitted at TA1 connector is left in place.