Your boiler has detected a critical internal error that prevents it from operating safely, so it has shut down to protect itself.

Boiler fault. Call engineer.

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring the temperature of your hot water fails to send a reliable signal to the main control board. The most common reason is that the sensor has electronically burnt out or developed a loose connection, meaning the boiler can no longer safely regulate heat and must shut down to prevent overheating.