Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Hinge down the PCB housing
Ensure gas is turned off at service valve
Attach pressure gauge to burner pressure test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

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 and checked for gas soundness.