Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Rotary Switch in DHW position
Keep Boiler Case Switch depressed
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the Red and Grey wires at the fan connector plug located at the top right hand side of the boiler.
The voltage should be approximately 80 volts AC.