Set Appliance State:
Mains Power Off
Mode selector switch to Off/Reset position
Set Multimeter to AC Volts 
Turn Hot Water Tap On
Hot water temperature control set to maximum
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Mode selector switch to DHW position.
Measure the voltage across live (brown wire) on connector M12 on the main PCB and a suitable neutral.
Measure the voltage across live (brown wire) on connector M12 on the main PCB and a suitable neutral.
Isolate as per TOPs P4
and refit as necessary