High
b8 on Viessmann Vitodens 300-W

Your boiler has stopped working because it can no longer safely monitor the temperature of the exhaust gases leaving 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

Burner blocked: Flue gas temperature sensor break

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

F7View →b4View →

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