77:1 Medium

Viessmann 77:1 Fault Code

Also written as 771 — same fault.

The Viessmann 77:1 fault code means: your boiler's internal communication system has detected an issue or a duplicate identity on its data network, which can sometimes happen after a power cut or part change.. It appears on 7 Viessmann models in our database.

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: LON user number

What causes the Viessmann 77:1 fault?

This fault occurs when the boiler's internal communication network becomes confused by seeing multiple components with the same identification number or when a signal is blocked. The most common reason is an electronic glitch following a power surge, or a conflict that arises after a technician has replaced a control board without reconfiguring its digital address.

Typical repair cost £100-250 (including labour)

DIY fix possible

Locate the 'Reset' button or menu option on the control panel.Perform a full system reset as per the user manual.Turn the power off at the fused spur switch for 30 seconds, then turn it back on.

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Viessmann models that show the 77:1 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann 77:1 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026