Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
DHW Demand
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the AC volts at the fan on the harness from the live to neutral (the neutral connection should be the blue on the fan connector).
Expected voltage should be between 160v and 240v.
Note:
Ensure a good connection is made.