Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Operating switch in Heating & 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:

Run the CH at maximum for several minutes.
Slowly turn the Temperature control to minimum.
Does the main burner flame modulate down or go out entirely?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.