High Lockout
9b:1 to 9b:127 on Viessmann Vitodens 333-F

Your boiler has detected an error with its internal temperature settings or the control unit, which may cause it to stop providing heating or hot water correctly.

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

Set temperature adjustable from 1 to 127°C

What causes this fault?

This fault occurs when the boiler's internal computer receives conflicting or impossible data regarding the temperatures it is supposed to maintain. It is usually caused by a communication failure between the main control board and the electronic coding plug that stores the boiler's specific settings. In some cases, damaged internal wiring or a failing circuit board can also lead to these incorrect readings.

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 guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on combis.

Read →

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 →