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 PCB

Measurement:

Test for continuity (zero resistance) of the two wires of the flow thermistor to pins 14 and 16 at connector X20.
Also inspect the harness for good overall
condition, no signs of shorting to the boiler casing etc.