Your boiler has lost connection with a temperature sensor that monitors the main water flow, likely due to a broken wire or a faulty sensor.

Controls without flow temperature sensor (low loss header): System flow temperature sensor lead break

What causes this fault?

This fault occurs when the boiler's control board can no longer detect a signal from the temperature sensor located on your system's piping. The most common reason is that the internal wiring has become damaged or the sensor itself has worn out and failed. Without this data, the boiler cannot accurately measure the water temperature leaving the unit, causing it to stop operating for safety reasons.