Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach pressure gauge to the inlet pressure test point
On/Off switch on
Gas On
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 and burner to ignite.
During the ignition sequence does the pressure gauge show an inlet pressure of at least 18mb or greater?
Note:
When "U" gauge is removed the test point should be tightened and checked for gas tightness.