BoilerFaultCodesViessmann 81:3 Fault Code
Also written as 813 — same fault.
The Viessmann 81:3 fault code means: your boiler is having trouble synchronizing its internal clock with an external control system, which might affect your heating schedules.. It appears on 5 Viessmann models in our database.
Technical description: Accept time via LON
What causes the Viessmann 81:3 fault?
This fault happens when there is a breakdown in communication between the boiler and other smart controllers or expansion modules connected via the LON network. It is usually caused by an incorrect setting in the boiler's menu or a conflict in how the devices are trying to synchronize their internal clocks. In most cases, it is a configuration error rather than a physical part failure.
DIY fix possible
Locate the main control panel and manually set the time and date.Restart the boiler system to re-establish the communication link.Check if any external thermostats or smart controls have lost power or connection.
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Gas Safe Register →Viessmann models that show the 81:3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 81:3 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