Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to AC Volts 
CH switch OFF
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Mains Power On
Measurement:
Pre-heat switch ON.
Measure voltage across connection D22 marked HI Fan (violet wire) and connection D23 marked N (neutral, grey wire) on 12 pin connector towards top L/H corner of main PCB 23.
Measure voltage across connection D22 marked HI Fan (violet wire) and connection D23 marked N (neutral, grey wire) on 12 pin connector towards top L/H corner of main PCB 23.
Isolate as per TOPs P4
and refit as necessary