Your boiler has lost communication with one of its internal heat sensors, meaning it cannot monitor temperatures accurately to operate safely.

Control mode: Sensor S3 lead break

What causes this fault?

This fault occurs when the boiler's control board loses its electrical connection to a specific internal temperature sensor. The most common cause is a failure of the sensor itself or loose, damaged wiring that prevents the boiler from safely monitoring the water temperature. Without this data, the system shuts down to prevent overheating.