Set Appliance State:

Gas Off
Air heater switch set to constant
Water heater set to OFF
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the ignition sequence to commence.
Does the combustion air fan run for longer than 30 seconds?
Note.
If the appliance has gone to lockout ensure this is reset before carrying out test.