Your boiler's temperature sensors have been swapped during installation or maintenance, causing the system to get confused about which pipe is heating up.

The heating flow thermistor and heating return thermistor are connected the wrong way round.

What causes this fault?

This fault occurs when the water inside the boiler heats up much faster than it can be moved away, leading to a dangerous heat buildup. The most common reasons are a failed pump that isn't pushing water round or a blockage in the heat exchanger, such as a buildup of sludge or limescale. In some cases, it may simply be that the internal sensors have become faulty and are providing incorrect temperature readings to the control board.