Medium Lockout
76:2 on Viessmann Vitodens 333-F

Your boiler is having trouble communicating with its internal control modules or multiple boiler system systems, which may stop it from operating correctly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

With KM BUS/cascade communication module; automatic recognition

What causes this fault?

This fault occurs when the boiler's brain loses contact with the internal communication modules that coordinate its heating tasks. The most common reason is a loose or damaged wiring connection between internal components, or a electronic failure within the communication module itself. Essentially, the different parts of the boiler can no longer 'talk' to each other to confirm it is safe and ready to fire up.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

76:1View →

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 →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →