Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Unplug connector CN4

Measurement:

Measure resistance across DHW Thermistor (purple wires).
This should be in the range 2K ohms (Hot) to 13K ohms (Cold).
Note:
Test to be carried out at disconnected connector.