Set Appliance State:

Mains Power Off
Mode selector switch to Off/Reset position
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Hot water temperature control set to maximum
Use insulation mat as necessary
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the voltage across the blue and brown wires at connector M14 on the main PCB.