High Lockout
D.34 on Vaillant ecoTEC plus Regular

Your boiler is having trouble controlling the speed of its internal fan, which is preventing it from starting up safely.

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 actual value

What causes this fault?

This fault occurs when the boiler's control board cannot accurately track or regulate how fast the fan is spinning. The most common reason is a mechanical failure within the fan motor or a communication break in the wiring, which prevents the boiler from safely clearing waste gases. Because the fan is a critical safety component, the system will prevent the boiler from firing up if the speed is inconsistent.

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 →

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