Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Disconnect the 5-pin electrical plug connecting the air pressure switch and fan
Withdraw the fan assembly by pulling forwards from the top edge
Disconnect fan venturi tube from air pressure switch,
Connect fga (+ve pressure test point) to disconnected fan venturi tube
Refit fan assembly, reconnecting the 5-pin electrical plug
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Ensure fan is running.
Measure fan pressure.
Expected pressure, positive greater than 1.0 mbar.