High Lockout
9C:0 on Viessmann Vitodens 300-W

Your boiler has detected an internal communication failure related to its electronic control system, preventing it from monitoring safe operation.

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

No monitoring

What causes this fault?

This fault occurs when the boiler's main control unit loses the specific digital 'identity' required to operate your heating system safely. It is typically caused by a corrupted coding chip or a breakdown in communication between the central computer and the internal sensors. Without these settings, the boiler shuts down as a safety precaution because it no longer knows its own technical parameters.

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

91:0View →bEView →

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