Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the spade connectors from the thermistor and the X20 connector from the RHS of the PCB

Measurement:

Test for continuity of the red and black wires, 5th and 7th down from top on RHS of connector and the end of the harness .
Also inspect the harness for good overall condition, no signs of shorting to the boiler casing etc.