Your boiler has detected that the water returning to the unit is much too hot, suggesting it isn't circulating properly through your radiators or pipes.

Return sensor above 95 °C

What causes this fault?

This fault occurs when water cannot move away from the boiler quickly enough, causing the temperature at the return pipe to spike dangerously high. The most common reasons are a seized internal pump failing to push the water or a major blockage in the pipework or heat exchanger that is strangling the flow. It can also be caused by a faulty sensor sending an incorrect high-temperature reading to the control board.