Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Attach U-gauge to gas inlet pressure test point
Ensure No demands
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if required.
With a standing pressure registered on the U-gauge, turn off the inlet gas supply isolation valve.
Apply a CH demand, allow the ignition sequence to commence.
During the sparking phase of ignition the gas pressure registered on the U-gauge should be seen to drop away, indicating that the gas valve has opened. (see note 1.)