Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to AC Volts 
Turn Hot Water Tap On
Boiler ON/OFF Switch OFF
CH Temperature Selector OFF
DHW Temperature Selector to Max
CH Temperature Selector OFF
DHW Temperature Selector to Max
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Boiler ON/OFF Switch ON.
Measure Mains Voltage across the Fan at connection 3 (live-white wire) and connection 1 (neutral-black wire).
Measure Mains Voltage across the Fan at connection 3 (live-white wire) and connection 1 (neutral-black wire).
Isolate as per TOPs P4
and refit as necessary