Your boiler has shut itself down as a safety precaution because it has been manually restarted too many times within a short period.

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What causes this fault?

This code appears when the boiler's internal computer is performing a software reboot, often triggered after the main power has been switched off and on or following a physical reset. It indicates that the control board is synchronizing its components and checking safety parameters before allowing the burner to ignite. In some cases, it can also appear if there is a momentary fluctuation in the electrical supply to your home.