BoilerFaultCodesViessmann 9A Fault Code
The Viessmann 9A fault code means: your boiler has lost communication with the sensor that monitors your solar heating panels, likely due to a damaged or disconnected wire.. It appears on 4 Viessmann models in our database.
Technical description: Control mode - Collector temperature sensor lead break
What causes the Viessmann 9A fault?
This fault occurs when the boiler's control system can no longer detect a signal from the temperature sensor located on your roof's solar panels. The most common reasons are a wire being damaged by wildlife, such as birds or squirrels, or a connection point becoming loose or corroded due to exposure to the elements.
This fault is often identified during the spring and summer months when homeowners notice their solar thermal system isn't contributing to their hot water production as expected.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Viessmann models that show the 9A fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 9A 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