BoilerFaultCodesViessmann 5A Fault Code
The Viessmann 5A fault code means: your boiler is struggling to detect that water is circulating through the system, which is preventing it from safely starting up to provide heating or hot water.. It appears on 3 Viessmann models in our database.
Technical description: Break, flow sensor
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 5A fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 5A 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