Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Disconnect the external controls from the main
terminal strip connections 2-3 and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

With external controls disconnected does the boiler still attempt to fire?
Note:
The appliance may not have external controls fitted (in this case the black wires will be fitted to terminals 1 and 3 not 1 and 2 as shown and no other wiring will be present) If no external controls are fitted then choose the "boiler still attempts to fire" outcome below.