Your boiler has detected that a key internal thermometer is broken or disconnected, meaning it cannot safely monitor the temperature of the water leaving the unit.

Temp.sensor fault: Flow temperature sensor defective or not connected.

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that measures the heat of the water being sent to your radiators. The most common reason is that the internal sensor has physically failed due to age or heat stress, though it can also be caused by loose wiring or dampness affecting the electrical connections.