A3:-9 to A3:15 Low

Viessmann A3:-9 to A3:15 Fault Code

Also written as A39TOA315 — same fault.

The Viessmann A3:-9 to A3:15 fault code means: your boiler is automatically managing its internal pump to protect the system from freezing based on the outdoor temperature.. It appears on 5 Viessmann models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Outside temperature below 1°C: Heating circuit pump ON; Outside temperature above 3°C: Heating circuit pump OFF

What causes the Viessmann A3:-9 to A3:15 fault?

This code appears when the outdoor temperature sensor detects that the air outside has dropped to near freezing levels. The boiler responds by automatically activating its internal pump to keep water moving through your pipes, which prevents them from freezing and potentially bursting. It is a built-in safety feature rather than a mechanical failure.

This notification will only appear during the colder winter months when external temperatures drop below 1°C.

DIY fix possible

Check that your outdoor temperature sensor is not covered by snow or debrisMonitor the boiler display to see if the message clears as the weather warms upNo action is required as this is an automated safety function

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Viessmann models that show the A3:-9 to A3:15 fault code

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

Source

Viessmann A3:-9 to A3:15 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