High Lockout
F64 on Glow-worm Flexicom hx

Your boiler's main internal computer has developed a communication or hardware error and cannot manage the heating process 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

Main PCB fault

What causes this fault?

This fault occurs when the boiler's central control board suffers an internal hardware failure or loses the ability to communicate with its sensors. The most common reason is age-related wear on the electronic components or a sudden electrical surge that has damaged the delicate circuitry.

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

F61View →F62View →F63View →F65View →F67View →

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 →