Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Gain access to PCB (use Insulation Mat as required)
Boiler On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the DC voltage between:
D15 (Purple wire) and D14 (Purple wire) on distribution board.
Voltage should be between 3 - 8V dc dependent on the Hot Water flow temperature.