Set Appliance State:

Gas Off
Water heater switch set to constant
Set thermostat to MID position
Air 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?
Note.
If the appliance has gone to lockout ensure this is reset before carrying out test.