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)
Disconnect connector A5 from the PCB
DHW taps OFF

Measurement:

Measure for resistance across connections 5 (green lead) and 6 (green lead) at the disconnected connector A5.
Expected resistance open circuit.