Medium Lockout
63:0 on Viessmann Vitodens 300-W

Your boiler is struggling to correctly communicate with its internal control system regarding how it should heat your hot water tank.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Without auxiliary function for DHW loading

What causes this fault?

This fault occurs when the boiler's main control board loses its electronic connection with the components responsible for heating your hot water cylinder. The most common reason is a breakdown in the communication module or a faulty sensor that prevents the boiler from knowing the water temperature in the tank. Without this data link, the boiler stops heating the cylinder as a safety precaution.

DIY fix possible

Turn the boiler's power switch off for 30 secondsSwitch the power back on and wait for the system to rebootCheck if the hot water function operates normally

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