Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
DHW Tap OFF
Disconnect the X2 connector from the PCB.

Measurement:

Measure the resistance between the blue and white wires on connector X2
Resistance should read open circuit as the tap is not on.