Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4

Observation:

Ensure that all the following system checks are completed:-
System/boiler is correctly charged as necessary.
System/boiler is bled of air.
Gas inlet pressure is correct.
Bypass (internal or external to the boiler) is correctly adjusted as necessary.
Ensure sufficient water supply to the appliance.