Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach temperature probes to the flow and
return pipes as indicated by the red arrows
Run the hot tap at a flow rate of 9.8 litres per minute
Attach a U gauge to burner pressure test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow DHW to run for a couple of minutes.
Expect a temperature differential of approximately 20°C or less?
Also monitor the burner pressure, it should remain steady and not modulate down prematurely before the hot water temperature is reached.
Is the differential as expected?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.