Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On

Observation:

Is Throttle Valve open sufficiently?
Open Throttle Valve fully then refer (if necessary) to manufacturer's instructions.
Pass this test if flow rate does not increase above 2.8 litres/min.
Fail this test if flow rate increases above 2.8 litres/min.