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 the DHW Thermistor (Purple wires).
This should be in the range 2 kOhms (Hot) to 13 kOhms (Cold).
Note:
Test to be carried out at disconnected connector.