Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of its hot water storage tank.

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What causes this fault?

This fault occurs when the boiler's control board receives a scrambled or impossible electrical signal from the sensor that monitors your hot water storage. The most common reason is that the sensor has failed internally or the protective coating on the wiring has worn through, causing an electrical 'short' against the metal casing. It can also be caused by water or moisture getting into the electrical plug, which disrupts the connection.