Set Appliance State:

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

Observation:

Boiler On/Off Switch On.
Allow fan to run.
Observe pressure gauge.
Does the gauge show a negative pressure of approximately 1.5 mbar?
Note:
When pressure gauge is removed pressure test point should be tightened & checked for gas soundness.