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 10 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.
- Viessmann Vitodens 050-W BPJD 35kW Combi Boiler manual (PDF)
- Viessmann Vitodens 100-W B1KA-35 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026