High Lockout
F1259 on Worcester Bosch CDi Classic

Your boiler's main internal computer or its configuration module has developed a communication failure, preventing the system from operating 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

Control box or the heat control module (HCM) is defective – HCM or control box connections need to be checked

What causes this fault?

This fault occurs when the boiler's main control board loses communication with the small memory chip that stores its specific operating settings. The most common reason is a physical connection failure or a component failure on the circuit board, which prevents the boiler from knowing how to safely manage the burner.

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

F0View →FdView →

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 →