Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
If mains voltage external controls are fitted (wired into "FT and "RT" at connector X1) remove them and make safe
If 24v controls are fitted to X17 then remove connector X17
Ensure NO demands are present
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the CH symbol still flash (CH demand)?
Note.
A new boiler out of the box or a new PCB will have a wire
link across the 24 volt external controls connections, this
should be removed unless appliance is fitted with EBUS controls.