High Lockout
F0 on Viessmann Vitodens 333

Your boiler has stopped working because its internal computer and burner system have detected a technical communication failure.

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

Burner blocked - Internal error

What causes this fault?

This fault occurs when the boiler's main control board loses the ability to communicate with the internal burner firing system. It is usually caused by a hardware failure within the electronic control unit (PCB) or a loose connection in the wiring harness that links these critical components. Because the computer cannot safely verify the burner's status, it shuts the system down to prevent an unsafe start-up.

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

F1View →F2View →

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 →