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
Temperature selector to high setting
Ensure hot water storage vessel is calling for heat
Boiler On/Off switch Off
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Boiler switch to On position.
Reset lockout if necessary.
Allow fan to run and ignition sequence to commence.
As the appliance goes through its ignition and run sequence, does the gas inlet pressure remain satisfactory (i.e. stays above 18 mb throughout the operation)?
Note:
When pressure gauge is removed pressure test point should
be tightened & checked for soundness.