BoilerFaultCodesViessmann A.11 Fault Code
Also written as A11 — same fault.
The Viessmann A.11 fault code means: your boiler has stopped working because the water pressure inside your heating system has dropped too low to operate safely.. It appears on 3 Viessmann models in our database.
Technical description: System pressure has fallen below the standard range
DIY fix possible
Locate the external filling loop, which is usually a flexible silver braided hose with one or two valves located underneath the boiler.Open the valves slowly to allow mains water to enter the system until the pressure gauge on the boiler front displays between 1.0 and 1.5 bar.Close the valves tightly and check for any visible leaks around the joints.
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Viessmann models that show the A.11 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann A.11 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
- Viessmann Vitodens 200-W B2HF System manual (PDF)
- Viessmann Vitodens 200-W B2KF Combi manual (PDF)
- Viessmann Vitodens 222-F B2TF manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026