High Lockout
F12 on Saunier Duval Isotwin F30E

Your boiler's internal computer has lost communication with the control panel or display, preventing the system from operating.

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 board connection fault

What causes this fault?

This fault occurs when the boiler's main brain and the front display panel stop talking to each other due to an electrical communication break. The most common reasons are a loose internal plug, moisture getting onto the electronics, or a component failure on one of the circuit boards themselves.

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

F13View →

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 →