Set Appliance State:

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

Observation:

Allow fan purge to complete (2 minutes).
Allow ignition sequence to commence.
Monitor the inlet pressure throughout the ignition sequence.
Does the pressure gauge show an inlet pressure of at least 18mb (37mb for LPG) throughout the operation?
Note:
When "U" gauge is removed test point should be tightened and checked for gas tightness.