BoilerFaultCodesViessmann 1A Fault Code
The Viessmann 1A fault code means: your boiler has shut down because it is struggling to discharge exhaust fumes safely or is detecting a significant lack of air flow.. It appears on 7 Viessmann models in our database.
Technical description: Burner blocked
What causes the Viessmann 1A fault?
This fault occurs when the boiler's internal sensors detect that exhaust gases cannot exit the building or fresh air cannot enter the system properly. It is usually caused by a heavy buildup of debris within the heat exchanger or a blockage in the flue pipe, which prevents the burner from firing safely. In some cases, a blocked condensate trap can also cause water to back up, effectively choking the combustion process.
This fault can be more frequent in autumn or during high winds, as fallen leaves or debris can obstruct the external flue terminal.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Viessmann models that show the 1A fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 1A 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 27 April 2026