BoilerFaultCodesViessmann 9A Fault Code
The Viessmann 9A fault code means: your boiler has lost communication with the temperature sensor that monitors the outdoor solar panels or the system collector, meaning it cannot accurately manage heat levels for those components.. It appears on 7 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 can no longer receive data from the sensor attached to your solar heating panels. The most common reason is a physical break in the cabling or a failed electrical component within the sensor itself, often caused by long-term exposure to the elements or loose connections. Since the boiler cannot see how hot the solar panels are, it stops using them to heat your water as a safety precaution.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Viessmann models that show the 9A fault code
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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.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026