BoilerFaultCodesViessmann b5 Fault Code
Also written as B5 — same fault.
The Viessmann b5 fault code means: your boiler has failed to complete its initial setup process, meaning it hasn't properly cleared air from the system or finished its first startup sequence.. It appears on 9 Viessmann models in our database.
Technical description: Installation in unfinished state - Boiler venting Fault in burner / Ignition valve
What causes the Viessmann b5 fault?
This fault occurs when the boiler's automatic commissioning program fails to complete, often because the system hasn't been fully bled of air during setup. It signifies that the internal sensors cannot confirm a stable flame or safe operating conditions, causing the computer to stop the sequence for safety. The root cause is usually a snag during the initial 'purging' phase or a component like the ignition electrode failing to spark correctly.
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 b5 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann b5 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