BoilerFaultCodesViessmann bF Fault Code
Also written as BF — same fault.
The Viessmann bF fault code means: your boiler is currently stuck in an automated filling process and cannot continue until this is resolved.. It appears on 7 Viessmann models in our database.
Technical description: Whilst filling is in progress (constant temperature control unit)
What causes the Viessmann bF fault?
This fault occurs when the boiler’s automated filling system becomes stuck or fails to complete its cycle, often because the system cannot reach the required pressure. The most common reason is a manual filling valve being left open by mistake or a malfunction in the electronic sensor that monitors the water level. In some cases, the internal control board may be unable to register that the filling process has finished, causing the boiler to halt for safety.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Viessmann models that show the bF fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann bF 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