BoilerFaultCodesViessmann b4 Fault Code
Also written as B4 — same fault.
The Viessmann b4 fault code means: your boiler's internal computer is having trouble processing electrical signals correctly, which is preventing it from running its safety and efficiency tests.. It appears on 6 Viessmann models in our database.
Technical description: Emissions test mode - Internal error analog converter
What causes the Viessmann b4 fault?
This fault occurs when the boiler's main brain fails to translate physical electrical signals into data it can understand. It is usually caused by an internal hardware failure within the circuit board's circuitry, meaning the boiler can no longer verify if it is burning fuel safely or efficiently. As a result, the system shuts down to prevent any unpredictable behavior.
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 →Avoid the next call-out bill
A cover plan would handle faults like b4: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Viessmann models that show the b4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann b4 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
- Viessmann Vitodens 242-F B2UA manual (PDF)
- Viessmann Vitodens 242-F FB2B manual (PDF)
- Viessmann Vitodens 300-W manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026