Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the main control box cover to expose the PCBs
Disconnect the fan speed control plug from the fan control PCB - this is the smallest of the plug connectors on the fan board and is nearest to you as you look at the board with the control panel down
Attach a pressure gauge to the outlet pressure test point on the gas valve
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Does the fan start to run at high speed and the gauge show a pressure of approximately 12 mb?
Note:
When pressure gauge is removed pressure test point should be tightened & checked for gas soundness.