BoilerFaultCodesViessmann 81:2 Fault Code
Also written as 812 — same fault.
The Viessmann 81:2 fault code means: your boiler is having trouble communicating with its external radio clock module, meaning it may not know the correct time or date for your heating schedule.. It appears on 5 Viessmann models in our database.
Technical description: The application of the radioclock module will be recognised automatically
What causes the Viessmann 81:2 fault?
This fault occurs when the boiler's main control unit loses its connection to the external radio clock receiver that synchronises the time. The most common reason is a loose wiring connection between the receiver and the boiler, or the internal circuitry of the radio module itself has failed. Because the boiler can no longer confirm the exact time, your programmed heating schedules may become inaccurate.
DIY fix possible
Locate the radio clock module (usually a small white box near the boiler) and ensure it is not blocked by metal objects.Restart the boiler using the power switch to trigger a fresh search for the signal.Access the 'Time and Date' settings via the control panel to manually set the time if the signal cannot be found.
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Gas Safe Register →Viessmann models that show the 81:2 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 81:2 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