Set Appliance State:

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

Measurement:

Allow the fan to run and boiler to attempt ignition.
Monitor the pressure from the fan, the pressure gauge will read negative and should reach a pressure of at least 0.8mb.
Repeat the test as necessary by resetting the appliance.
Note:
When pressure gauge is removed pressure test point
should be tightened & checked for gas soundness.