Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Note:
The thermistors are located in the flow and return header to the upper right of the heat exchanger. The thermistor nearest the front is the flow thermistor and should be connected by red leads

Observation:

Check that the sensors are correctly connected.
The green leads are connected to the return temperature sensor (sensor farthest back) and the red leads are connected to the flow temperature sensor (sensor nearest the front).
Ensure that the connections are sound at both the sensors and the PCB.