High Lockout
D.33 on Vaillant ecoTEC plus Regular

Your boiler's fan is not reaching the correct speed required to safely clear exhaust gases, preventing the heater from starting up.

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

Fan speed target value

What causes this fault?

This fault occurs when the boiler's control system attempts to start the fan, but the fan fails to spin at the required speed to safely clear waste gases. The most common reasons include a mechanical failure of the fan motor, a buildup of debris blocking the rotation, or a communication error from the main circuit board. Because the boiler cannot guarantee a safe flow of air, it will refuse to ignite 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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Related fault codes

F.32View →F.33View →

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