Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Main switch in ON position
Hot Water Control Thermostat set to Maximum
Attach U-gauge to burner pressure test point
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Is burner pressure greater than 1.5 mbar?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.