BoilerFaultCodesTechnical description
There is excessive temperature difference between the flow and return thermistors.
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.
This fault is more frequent in autumn when heating systems are first turned on after summer, as pumps can seize during long periods of inactivity.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.