Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
DHW Demand
Move selector switch from Gravity position to Fully Pumped position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

With the selector switch moved to the
fully pumped position does the fan now run?
Important:
If fan still does not run keep the selector switch in Fully Pumped position.
Remember to return it to Gravity setting after diagnosis is complete.