BoilerFaultCodesViessmann 65:1 Fault Code
Also written as 651 — same fault.
The Viessmann 65:1 fault code means: your boiler is struggling to direct heated water to either your radiators or your hot water taps because a motorized internal valve has become stuck or failed.. It appears on 5 Viessmann models in our database.
Technical description: Diverter valve by Viessmann
What causes the Viessmann 65:1 fault?
This fault occurs when the internal motor that moves the diverter valve fails to reach its intended position, often because the valve has become stuck or stiff. This is usually caused by a buildup of debris or sludge from the heating system water getting inside the moving parts, or eventually, the motor simply wearing out over time.
This issue is frequently noticed in autumn when homeowners switch their heating back on for the first time, putting strain on a valve that has remained stationary all summer.
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 65:1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 65:1 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