BoilerFaultCodesVaillant d.91 Fault Code
Also written as D91 — same fault.
The Vaillant d.91 fault code means: your boiler is currently trying to synchronize its internal clock with a radio signal from a time transmitter, which is a standard background process and does not affect your heating or hot water.. It appears on 8 Vaillant models in our database.
Technical description: Status DCF77
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Vaillant models that show the d.91 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant d.91 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
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Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026