Your boiler has lost communication with the temperature sensor in your hot water tank because of an electrical fault, meaning it cannot accurately monitor your solar heated water.

Control mode - Solar: Cylinder temperature sensor shorted out

What causes this fault?

This fault occurs when the electrical circuit for the solar hot water sensor is completed prematurely, usually due to moisture ingress or damaged insulation. The most common reason is that the sensor probe has failed internally or the wires connecting it to the boiler have become pinched or corroded, preventing the system from reading the water temperature.