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.