Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
Turn Hot Water Tap On
Gain access to PCB (use Insulation Mat as required)
Selector switch to DHW and CH position
CH temperature selector to Max
DHW temperature selector to Max
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Place positive probe on connection 2 (brown wire) and negative probe on connection 3 (blue wire) at connector X3 on the main PCB.
Turn the DHW inlet isolator OFF.
Observe multimeter, mains voltage should be present.
Turn the DHW inlet isolator ON.