Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Selector switch to Hot Water
Gain access to PCB (use insulation mat as necessary)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across connection 3 (white wire) on connector marked X400 and Earth.
Expect approximately 7 volts DC.
Turn hot water tap ON with a flow rate of 2 l/min
Measure voltage across connection 3 (white wire) on connector marked X400 and Earth.
Expected voltage volts should drop to 0 (zero) volts DC.