Isolate Mains Power to appliance as per TOPs P4
Please ensure the following system checks are completed

Observation:

System/Boiler is correctly charged as necessary.
System/Boiler is bled of air.
Gas inlet pressure is correct.
Sufficient supply of Mains Cold Water
Bypass (internal or external to the boiler) is correctly adjusted as necessary.
Manual reset devices are set.
That an adequate cold water supply is available to the appliance.