Your boiler has lost the ability to measure the temperature of your hot water properly, which usually means your taps will only run stone cold or may fluctuate wildly in temperature.

Exact technical description from manual

What causes this fault?

This fault occurs when the sensor responsible for monitoring your hot water temperature fails to send a reliable signal to the boiler's main control board. The most common reason is that the internal component of the sensor has degraded over time or suffered a short circuit, preventing the boiler from knowing how hot to heat your water. In some cases, the issue is caused by loose or corroded electrical connections between the sensor and the boiler.