BoilerFaultCodesViessmann A9 Fault Code
The Viessmann A9 fault code means: your boiler is having trouble 'talking' to its internal pump, which means it cannot flow water around the system to heat your home.. It appears on 7 Viessmann models in our database.
Technical description: Installation accessories- Communication with variable speed pump (low voltage HM)
What causes the Viessmann A9 fault?
This fault occurs when the boiler's main control board loses its digital connection with the internal circulation pump. The most common reason is a hardware failure within the pump’s electronic motor head or a loose communication cable, meaning the boiler can no longer control the speed of the water flow. Without this connection, the system shuts down as a safety precaution to prevent the heat exchanger from overheating.
This fault is more common in autumn when the boiler is switched back on after being idle all summer, which can cause internal pump components to seize or fail.
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 A9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann A9 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