High Lockout
F9 on Keston S30

Your boiler's internal computer has encountered a critical error and cannot communicate with the rest of the system.

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

Printed circuit board (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 send signals to other components. It is often caused by a surge in the electrical supply, moisture ingress, or simply the natural degradation of the solder joints and microprocessors over time. The system shuts down as a safety precaution because it can no longer monitor the boiler's operation accurately.

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

F7View →L2View →

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 →