BoilerFaultCodesViessmann 9F Fault Code
The Viessmann 9F fault code means: your boiler is struggling to communicate with the solar panel system used to help heat your water.. It appears on 7 Viessmann models in our database.
Technical description: Control mode - Solar control unit error message
What causes the Viessmann 9F fault?
This fault occurs when the boiler's main brain loses its connection with the separate control box that manages your solar thermal panels. The most common reason is a loose or damaged communication cable between the two units, or a component failure within the solar pump station itself. When this happens, the system can no longer tell if the solar panels are providing heat, so it stops relying on them to warm your water.
This issue is often more noticeable during the spring and summer months when high levels of sunshine mean the boiler should be relying heavily on solar power rather than gas.
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 9F fault code
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Source
Viessmann 9F 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