Medium Lockout
73:7 on Viessmann Vitodens 300-W

Your boiler has detected a communication error with its internal control system or a connected smart module, which may prevent it from managing your heating correctly.

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

Constantly ON

What causes this fault?

This fault occurs when the boiler's internal communication hardware suffers a permanent electronic failure, meaning it can no longer send or receive data from the control system. The most common reason is a short circuit or component degradation within the LON module or the main control board itself, which prevents the boiler from identifying its operating instructions. Because the system can no longer 'talk' to its sensors and switches, it shuts down for safety.

Engineer required

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

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