Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove wires / link from external controls terminal connector and make safe
Turn selector switch back to heating & hot water
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow self-check to complete.
Does the boiler still fire with external controls / link disconnected?