BoilerFaultCodesViessmann 65:3 Fault Code
Also written as 653 — same fault.
The Viessmann 65:3 fault code means: your boiler is struggling to switch the flow of heated water between your radiators and your hot water taps because of an internal valve malfunction.. It appears on 5 Viessmann models in our database.
Technical description: Diverter valve by Grundfos
What causes the Viessmann 65:3 fault?
This fault occurs when the internal motor fails to move the valve into the correct position to send heat where it is needed. The most common reason is that the mechanical components inside the valve have become stuck due to a buildup of debris or wear and tear, preventing the motor from completing its cycle. This results in the boiler being unable to differentiate between heating your home and warming your water.
This fault is more frequent during the autumn and winter months when the boiler is regularly switching between heating and hot water demands after a period of summer inactivity.
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 65:3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 65:3 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