Set Appliance State:
Isolate Mains Power to appliance as per TOPs P4
Remove the connector J1 from the PCB
Remove the connector J1 from the PCB
Measurement:
Ensure Thermistors are secured tightly to the pipework.
Measure the resistance across the blue and white (5 & 6) wires of the connector.
Measure the resistance across the blue and white (5 & 6) wires of the connector.

Isolate as per TOPs P4
and refit as necessary