BoilerFaultCodesViessmann A4 Fault Code
The Viessmann A4 fault code means: your boiler has detected that the water pressure inside your central heating system has become too high, which can put unnecessary strain on the internal components.. It appears on 2 Viessmann models in our database.
Technical description: Max. system pressure exceeded
DIY fix possible
Locate a radiator in your home that has a bleed valvePlace a cloth and container under the valveSlowly open the valve to let a small amount of water out until the pressure gauge on the boiler drops back into the green zoneEnsure the valve is securely closed once the pressure has normalized
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Viessmann models that show the A4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann A4 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