Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Gain access to PCB (use Insulation Mat as required)
Disconnect mains voltage external control wiring, leave in a safe place
Link out external controls at X20 on the PCB (wire link)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow self-check to complete.
Does the boiler now respond to a CH demand?
Note:
If boiler still does not respond, leave external controls
linked out for next tests, remember to re-instate any
external controls once a diagnosis has been made.