High Lockout
F27 on Glow-worm Flexicom hx

Your boiler has stopped working because it cannot confirm that the flame is staying lit 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

Flame detection fault

What causes this fault?

This fault occurs when the boiler's control system senses a flame when it shouldn't, or it cannot verify that the gas has ignited properly. The most common reason is a sensor or electrode that has become worn, dirty, or damaged, leading to an incorrect signal being sent to the main circuit board.

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

F28View →F29View →

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 →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →