Your boiler's hot water sensor has stopped working, so the system is unable to tell how hot your water is and has stopped heating it for safety.

Hot water NTC (Negative Temperature Coefficient) error

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring your hot water temperature fails or sends an incorrect signal to the main control board. The most common reason is that the sensor has physically degraded over time or developed a short circuit, meaning the boiler can no longer safely verify how hot the water is. Occasionally, loose or damaged wiring between the sensor and the boiler’s computer can also trigger this shutdown.