High Lockout
F20 on Glow-worm Xtramax HE

Your boiler's computer components are having trouble communicating with each other because they are running incompatible software versions.

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

Software incompatibility.

What causes this fault?

This fault occurs when the main control board and the display panel are unable to talk to each other correctly because their internal programs do not match. This usually happens after a repair where a new part has been installed that is running a different software version than the original components. It essentially means the boiler’s 'brain' and its 'voice' are no longer speaking the same language.

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 guides you might find useful

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 →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →