Your boiler has detected a problem with the temperature sensor that monitors the water returning to the unit, causing it to stop running for safety.

Sensor fault S2

What causes this fault?

This fault occurs when the boiler's internal computer stops receiving a reliable temperature reading from the sensor on the return pipe. It is usually caused by the sensor itself failing, or a loose connection in the wiring that prevents the signal from reaching the control board. For safety, the boiler shuts down because it can no longer monitor if the water returning from your radiators is at a safe temperature.