High Lockout
6C:0 to 6C:100 on Viessmann Vitodens 300-W

Your boiler's internal pump or fan is not running at the speed required to circulate water or air correctly through the system.

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

Set speed adjustable from 0 to 100%

What causes this fault?

This fault occurs when the boiler's control board sends a command to the pump or fan but receives a confused or incorrect speed signal in return. It is essentially a communication breakdown where the internal computer cannot confirm that these vital moving parts are spinning at the correct rate to safely manage heat and airflow.

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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