Set Appliance State:

On/Off Switch OFF
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Attach pressure gauge to burner pressure test point
External controls calling for heat
CH temperature switch to minimum
DHW temperature switch to maximum
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Turn On/Off switch to On.
Allow fan to run and burner to ignite.
Does the pressure gauge show a burner pressure as per data badge?
The appliance is range rated and burner pressure can be 2.5 to 11.8 mb.
Note: When pressure gauge is removed pressure test point should be tightened & checked for gas soundness.