Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of the water returning to the unit, causing it to shut down for safety.

Return sensor short circuit – check connections to return sensor

What causes this fault?

This fault occurs when the boiler's control board receives an incorrect electrical signal from the sensor that monitors the temperature of the water coming back from your radiators. The most common reason is that the sensor has suffered an internal electrical failure or the wiring connecting it to the main board has become damaged or loose.