Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Attach pressure gauge to inlet pressure test point on gas valve
The inlet pressure test point is the left hand of the two ports
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Does the pressure gauge show an inlet pressure of 18mb or greater throughout the operation?
Note:
When pressure gauge is removed pressure test point should be tightened & checked for gas soundness.