Set Appliance State:

Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
CH Selector Switch OFF
Gas Control Knob in Main Burner position
Attach a U Gauge to the Venturi Air Signal Test Point at the Gas/Air Ratio Unit Pipe
Gas On; Pilot On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

With the DHW tap fully open, does the U Gauge indicate the air pressure at approximately 2.5 mb?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.