Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach FGA to gas valve pressure outlet test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if required.
Allow the boiler to commence its ignition sequence.
With the fan running measure the fan pressure, the reading will be a negative pressure.
Expect a pressure of at least -1.5mb or greater (e.g. -3mb)
Is the fan pressure satisfactory?
Notes:
1. It may take approximately 40 seconds for the boiler to recognise a demand and for the fan to run, this will depend upon the boiler status. (Status code guidance CLICK HERE)
2. When the FGA is removed the pressure test point should be tightened and checked for gas tightness.