Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach a U-gauge to the gas valve outlet pressure test point
Selector switch to DHW and CH position
CH temperature selector to Max
DHW temperature selector to Min
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow burner to ignite and ramp up to full pressure.
Slowly turn the CH Temp selector to Min position.
Does the gas pressure decrease as the burner modulates?
Note:
When U-gauge is removed pressure test point should be tightened and checked for gas soundness.