BoilerFaultCodesViessmann A3 Fault Code
The Viessmann A3 fault code means: your boiler has switched itself off because a sensor inside detects that the exhaust system or internal temperature isn't configured exactly as it should be.. It appears on 3 Viessmann models in our database.
Technical description: Burner blocked, flue gas temperature sensor not positioned correctly
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Viessmann models that show the A3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann A3 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
- Viessmann Vitodens 100-W manual (PDF)
- Viessmann Vitodens 242-F B2UA manual (PDF)
- Viessmann Vitodens 242-F FB2B manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026