High Lockout
D.34 on Vaillant ecoTEC

Your boiler is struggling to accurately measure how fast its internal fan is spinning, which is preventing the system from heating correctly.

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 get a consistent reading of how fast the fan is spinning. The most common reason is a communication failure between the electronic heart of the boiler and the fan motor, often due to loose wiring or a failing motor.

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.

Find a Gas Safe engineer
Search the official register →

Related fault codes

F.32View →D.33View →

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →