Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
Isolate Mains Power to appliance as per TOPs P4
Selector Switch OFF
Gas Control Knob in Main Burner position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

After 30 seconds of operation measure the Slow Fan voltage across connections Fan L (Red) and Fan N (Purple) on connector CN2.
This should be in the range 160 - 180 V AC.