What does the Viessmann Vitodens 100-W F91 fault code mean?
The F91 fault code on the Viessmann Vitodens 100-W indicates a problem with the boiler’s energy efficiency, often related to the combustion process or sensor issues. This fault can lead to reduced performance and increased energy consumption.
Common Causes
- Faulty Lambda Sensor: The lambda sensor measures the oxygen content in the flue gases; if it fails, it can cause incorrect combustion readings.
- Blocked Flue or Air Intake: A blockage in the flue or air intake can restrict airflow, leading to inefficient combustion and triggering the fault code.
Can I fix it myself? (DIY)
It is highly recommended to call a Gas Safe Engineer for this issue, as it involves gas and combustion systems that require professional expertise. Attempting to fix it yourself could be dangerous.
Estimated Repair Cost (2026 UK Prices)
The estimated repair cost for resolving the F91 fault could be around £150 to £300, depending on whether the lambda sensor needs replacement or if there are blockages that require clearing. This cost reflects the complexity of diagnosing and fixing energy efficiency issues.