High Lockout
73:7 on Viessmann Vitodens 333-F

Your boiler communication module has a permanent fault that is preventing the system from talking to its controls correctly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely 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 is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

73:0View →

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