BoilerFaultCodesViessmann A.21 Fault Code
Also written as A21 — same fault.
The Viessmann A.21 fault code means: your central heating system currently has too much water inside it, causing the internal pressure to rise above the safe limit.. It appears on 2 Viessmann models in our database.
Technical description: System pressure has exceeded maximum pressure.
DIY fix possible
Locate a radiator in your home, preferably upstairs.Place a cloth and a container underneath the radiator bleed valve.Use a radiator key to open the valve slightly and release water until the pressure gauge on the boiler drops to between 1.0 and 1.5 bar.
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Viessmann models that show the A.21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann A.21 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