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

AUX return sensor failure

What causes this fault?

This fault occurs when the boiler's internal computer receives an incorrect or missing signal from the sensor that monitors water coming back from your radiators. This is usually caused by the sensor itself burning out or developing a physical defect, but it can also happen if the internal wiring has become loose or damaged. Because the boiler cannot accurately track the water temperature, it shuts down to prevent the system from overheating.