Your boiler has lost the ability to read the temperature of the water returning to it, so it has safely shut down to prevent overheating.

Open circuit on return sensor

What causes this fault?

This fault occurs when the boiler's control board can no longer read the temperature of the water coming back from your radiators. It is usually caused by the internal sensor failing due to age or a loose electrical connection between the sensor and the main control unit. Because the boiler cannot monitor its own temperature, it shuts down as a safety precaution to prevent overheating.