Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the wring from the external controls terminals 1 - 3,
noting their locations and make safe
Fit a wire link between terminals 1 and 3
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the boiler now respond to a CH demand?
Note:
Regardless of test outcome do not refit wiring until diagnosis is complete.