Set Appliance State:

Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Operating switch in Water position
Temperature control to Max
Connect U gauge to burner pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Open hot water tap and ensure burner ignites at maximum gas pressure. Run burner for several minutes until burner modulates, (modulation stage will be reached sooner if water flow is reduced at Hot tap).
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.