High Lockout
FA on Viessmann Vitodens 050-W

Your boiler has failed to start properly and the fan is running constantly to clear out the system, preventing the heater from firing 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

Burner in fault state. Fan stop not achieved. Check the fan, check the fan cables, check the fan control.

What causes this fault?

This fault occurs when the boiler's internal computer detects that the fan is spinning at the wrong speed or failing to stop when it should. Because the fan is responsible for clearing out dangerous exhaust gases, the system enters a safety mode and prevents the burner from lighting. The most common reason is a communication error between the fan motor and the main control board, often caused by worn-out electrical components.

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

F4View →F5View →F9View →

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 →