Set Appliance State:

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

Observation:

Allow ignition to commence.
Does the inlet pressure remain constant and above a minimum of 18 mbar?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.