A.12 Low

Viessmann A.12 Fault Code

Also written as A12 — same fault.

The Viessmann A.12 fault code means: your boiler's internal clock has lost power because its small backup battery has run out, which might affect your timed heating schedules.. It appears on 4 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: Real time clock battery flat — replace CR2032 in HMU
Typical repair cost £5-15 (including labour)

DIY fix possible

Locate the control unit on the front of the boilerOpen the control unit housing to access the Heat Management Unit (HMU) circuit boardCarefully remove the depleted CR2032 coin cell battery and replace it with a new oneReassemble the unit and reset the time and date on the display

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Viessmann models that show the A.12 fault code

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

Source

Viessmann A.12 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 7 July 2026