Set Appliance State:

CH Demand
Isolate Mains Power to appliance as per TOPs P4
Selector switch to heating and hot water position
CH temperature selector to MAX
DHW temperature selector to MIN
Attach U-gauge to gas valve outlet pressure test point
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to fire.
Does the burner pressure increase to its set pressure after approximately 90 seconds?
Does the CH system begin heating correctly?