Medium Lockout
98:1 to 98:5 on Viessmann Vitodens 333-F

Your boiler is confused because it doesn't know which type of heating system it is supposed to be controlling.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

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

System number adjustable from 1 to 5

What causes this fault?

This fault occurs when the boiler's internal memory loses track of the specific heating layout it is supposed to manage, such as whether it is heating just radiators or a hot water cylinder too. The most common reason is a communication error between the main control board and the coding plug, which acts like a small identity chip for the appliance. Without this clear 'identity,' the boiler stops operating because it no longer knows which internal components to activate.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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