Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Adjust DHW flow rate to approx. 6 litres/min
DHW Temperature Control set to MAX
Attach pressure gauge to gas valve pressure outlet test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow burner to ignite and establish.
After approximately 45 seconds turn the DHW potentiometer down towards minimum, does the burner pressure start to modulate down as the temperature setting decreases?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.