Your boiler has sensed that the water returning from your radiators is hotter than the water leaving the boiler, which usually means there is a circulation problem or a sensor fault.

Return sensor temperature > flow sensor temperature.

What causes this fault?

This fault occurs when the boiler's internal sensors detect that the system is operating in reverse, with the return pipe being hotter than the flow pipe. The most common reason is a circulation failure, often caused by a faulty pump or a blockage in the heat exchanger that prevents water from moving through the boiler correctly. It can also be triggered if one of the temperature sensors has failed and is providing an incorrect reading to the control board.