BoilerFaultCodesViessmann 79:1 Fault Code
Also written as 791 — same fault.
The Viessmann 79:1 fault code means: your boiler's electronic brains are experiencing a communication conflict or an internal memory error, which may cause the system to stop working correctly.. It appears on 5 Viessmann models in our database.
Technical description: Control unit is fault manager
What causes the Viessmann 79:1 fault?
This fault occurs when the boiler's internal computer chips stop being able to talk to each other reliably across the main circuit board. It is usually caused by a physical degradation of the electronic soldering or a failure in the communication module that acts as the 'translator' for the system's instructions. When these signals are interrupted, the boiler shuts down because it can no longer safely coordinate the fan, pump, and burner.
DIY fix possible
Locate the power switch or fused spur for the boilerTurn the power off for 30 secondsTurn the power back on and wait for the system to reboot
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Gas Safe Register →Viessmann models that show the 79:1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 79:1 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