BoilerFaultCodesViessmann bA Fault Code
Also written as BA — same fault.
The Viessmann bA fault code means: your boiler is having trouble communicating with the control kit that manages the heating for your secondary radiator or underfloor heating zone.. It appears on 7 Viessmann models in our database.
Technical description: Communication fault - extension kit for mixer circuit M2. Mixer M2 continues to move
What causes the Viessmann bA fault?
This fault occurs when the boiler's main brain loses its connection with the electronic controller that manages your underfloor heating or a specific radiator zone. The most common cause is a wiring issue between the units or a component failure within the secondary control kit, which prevents the boiler from knowing how to correctly adjust the water temperature for that zone.
This issue is more frequently noticed during the transition into autumn or winter when the heating is first switched on and these secondary circuits are required to operate for the first time in months.
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 bA fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann bA 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