Your boiler has lost connection with the sensor that monitors the temperature of your hot water cylinder, meaning it cannot heat your water correctly.

Control mode - Cylinder 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 probe that monitors your hot water tank. The most common reason is that the internal wiring has become damaged or loose, or the sensor itself has suffered an electrical failure due to age. Because the boiler doesn't know how hot the water is, it stops heating the cylinder as a safety precaution.