Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set DHW temperature selector to MAX
Attach pressure gauge to the inlet pressure test point
Gas ON
Selector Switch to Hot Water and Heating
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow fan to run and ignition sequence to commence.
Does the pressure gauge show an acceptable minimum
gas inlet pressure of at least 17mb throughout its operation?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.