Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach pressure gauge to gas inlet pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if required.
Allow self-check to complete and ignition sequence to commence.
Does the pressure gauge show an acceptable minimum gas inlet pressure throughout its operation?