Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Locate the Flow Thermistor and remove the two connectors
(one with Black wires and one with a Red).

Measurement:

Test resistance between two pins of the thermistor.
The resistance will depend on temperature and should conform to the table below:
Temperature °CResistance in kOhms
20 - 3012.0 - 8.2
30 - 408.5 - 5.5
40 - 505.8- 3.8
50 - 604.0 - 2.7
60 - 702.9 - 2.0
70 - 802.1 - 1.4

Refit the connector after the test.