Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Remove combustion chamber
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Approximately 8 seconds after mains power is turned on, measure the voltage between the black and blue wires at the fan.
Mains voltage expected.