High Lockout
b0 on Viessmann Vitodens 333

Your boiler has stopped working because it has detected a technical fault with the sensor that monitors the heat of the exhaust gases.

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 short circuit

What causes this fault?

This fault occurs when the boiler's internal computer receives an incorrect electrical signal from the sensor that monitors the temperature of the exhaust fumes. The most common reason is that the sensor itself has suffered an internal failure or the electrical wiring connecting it to the main control board has become damaged or loose. Because the boiler can no longer safely monitor how hot the waste gases are, it shuts down immediately as a safety precaution.

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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