77:2 to 77:99 Medium

Viessmann 77:2 to 77:99 Fault Code

Also written as 772TO7799 — same fault.

The Viessmann 77:2 to 77:99 fault code means: your boiler's internal communication system has identified an addressing conflict between different control units on the network.. It appears on 5 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, adjustable from 1 to 99: 1-4=boiler, 5=cascade, 10-...=Vitotronic 050, 99=Vitocom

What causes the Viessmann 77:2 to 77:99 fault?

This fault occurs when two or more components on the boiler's internal communication network are trying to use the same identification number. It is usually caused by a technician incorrectly configuring the settings during a repair or by a digital glitch that has scrambled the control unit's memory. Because the system cannot distinguish between the different parts, it struggles to send and receive commands correctly.

Typical repair cost £90-180 (including labour)

Engineer required

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

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Viessmann models that show the 77:2 to 77:99 fault code

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

Source

Viessmann 77:2 to 77:99 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