Your boiler has lost communication with a sensor that monitors the temperature of the water flowing into your heating system, usually because of a loose or damaged wire.

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 the main pipework. The most common reason is a physical break in the internal wiring or a sensor that has failed due to age, preventing the boiler from knowing how hot the water is as it leaves the unit. Because the system cannot safely monitor the heat, it shuts down to prevent damage.