Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove bottom facia panel
Disconnect connector X4 from the PCB marked FAN

Measurement:

Test 1:
Measure the resistance between the yellow and brown wires on X4.
For 24kW fan the resistance should be approximately 60 Ohms.
For 28kW fan the resistance should be approximately 45 Ohms.
Test 2:
Measure the resistance between the
brown wire on X4 to a suitable earth point.
For both 24kW and 28kW fans the
resistance should be greater than 1M Ohm.