High Lockout
F.68 on Vaillant ecoTEC sustain

Your boiler is struggling to keep its flame burning steadily, causing it to shut down for safety.

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

Unstable flame signal

What causes this fault?

This fault occurs when the boiler's control system detects that the flame is flickering or dropping out during operation. It is often caused by a worn sensing probe that can no longer 'see' the flame effectively, or a gas valve that isn't providing a steady enough flow of fuel to keep the burner alight.

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.27View →F.28View →F.29View →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Pilot light keeps going out

Older boilers — thermocouple, draught, gas supply.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →