Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect Red wires from connections 7 & 8 on the upper connector block
(Ensure other end of disconnected wires are connected to the DHW sensor)
Ensure DHW is cool

Measurement:

Measure resistance between the two Red wires.
Note:
Resistance measured will vary according to the boiler temperature, the cooler the boiler the higher the resistance.
The resistance should conform to the table below.
Temperature °CResistance in kOhms
10-2019.9-12.4
20-3012.4-7.9
30-407.9-5.2
40-505.2-3.5
50-603.5-2.4
60-702.4-1.7
70-801.7-1.2