Set Appliance State:

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

Observation:

Boiler switch ON.
Reset as necessary.
Allow fan to start.
Does the fan run and the pressure gauge show a pressure of 1.5 mb?
Note:
When pressure gauge is removed pressure test point should be tightened & checked for soundness.