Set Appliance State:

Gas On; Pilot On
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Selector switch to HOT WATER position
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:

Observe U-gauge.
Does the burner cross light from pilot and go to maximum burner approximately 15.3 mb?
Note:
When U-gauge is removed, test point should be tightened and checked for gas soundness.