Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Control Thermostat in high position
High/Low Switch in Low position
Connect a U-Gauge to the gas valve burner pressure test point
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the Boiler to purge itself and the Burner to ignite.
Does the U-Gauge indicate a pressure approx. to the table below;
Does the High/Low Switch Low Flame Indication Neon (Green) illuminate?