High
C6 on Viessmann Vitodens 100-W

Your boiler's fan or airflow system is failing to start up properly, which means the unit is preventing itself from igniting to stay safe.

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

Burner blocked: Differential pressure sensor fault during start-up or fan fault

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

F4View →F7View →

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