Your boiler has lost its ability to monitor the temperature of the stored hot water, meaning it cannot heat your water reliably or safely.

Storage vessel thermistor/wiring fault

What causes this fault?

This fault occurs when the sensor responsible for monitoring the temperature inside the boiler's internal hot water storage tank fails or loses its connection. The most common reason is that the sensor has physically degraded over time or the electrical wiring has become loose or corroded, preventing the control board from knowing if the water is hot or cold.