High Lockout
Fd on Viessmann Vitodens 333

Your boiler's internal computer has detected a problem with the burner's ignition system and has shut down as a safety precaution.

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. Burner control unit fault. Cause: Check the ignition electrodes and leads. Check whether a strong interference (EMC) field exists near the equipment. Press E. If the fault is not removed, replace the control unit.

What causes this fault?

This fault occurs when the boiler's internal computer receives an inconsistent signal from the ignition system, leading it to crash or freeze for safety. The most common reason is a hardware failure within the control board itself, or electrical interference that prevents the burner from firing up correctly.

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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