Set Appliance State:

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

Observation:

Reset lockout as necessary.
Allow fan to start and ignition sequence to run.
Observe pressure gauge.
Does the inlet pressure remain above 18mb (37mbar for LPG)
throughout the ignition sequence?
Note:
When pressure gauge is removed pressure test point should be tightened and checked for gas soundness.