Your boiler has detected a technical fault with the sensor that monitors water temperature returning to the unit, causing it to shut down for safety.

Short circuit on return sensor

What causes this fault?

This fault occurs when the internal control board receives an electrical signal from the return sensor that is outside its normal operating range. The most common reason is that the sensor's internal wiring has failed or a physical break in the connection has occurred, preventing the boiler from accurately monitoring the temperature of the water coming back from your radiators.