Set Appliance State:

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

Observation:

Allow self-check to complete.
Turn CH demand ON.
Allow fan to run and ignition sequence to commence.
Observe the U-gauge. The inlet pressure should remain at or above 20mbar (37mb for LPG) throughout boiler ignition sequence.
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.