Set Appliance State:

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

Observation:

Reset the appliance if required.
(Press reset button once then release)
Allow the fan to run and ignition sequence to commence.
Does the pressure gauge show an acceptable minimum
gas inlet pressure throughout its operation?
Note.
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.