High Lockout
b8 on Viessmann Vitodens 333

Your boiler has shut down because it cannot accurately measure the temperature of the exhaust gases, causing the burner to stop for safety.

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

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that monitors the heat of the exhaust gases. The most common reason is a physical break in the sensor's internal circuitry or a loose connection in the wiring harness. For safety, the boiler shuts down because it can no longer verify that the exhaust temperatures are within safe limits.

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

B7View →F9View →

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