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 CN4

Measurement:

Measure resistance across DHW Thermistor (purple wires).
This should be in the range 2K ohms (Hot) to 13K ohms (Cold).