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 plug from flow thermistor (located at the top back of the heat exchanger)
Disconnect connector LV1 from the PCB

Measurement:

Check for continuity (zero resistance) of each of the
wires, in turn, running between the red wires on LV1
(pins 6 and 7) to the flow thermistor.
Also inspect the harness for good overall condition.