BoilerFaultCodesVaillant D.91 Fault Code
Also written as D91 — same fault.
The Vaillant D.91 fault code means: your boiler is checking for a radio signal to automatically set the time and date using an outdoor weather sensor.. It appears on 5 Vaillant models in our database.
Technical description: DCF status with connected external probe
What causes the Vaillant D.91 fault?
This status appears when the boiler is attempting to synchronize its internal clock and calendar with a radio signal from an external weather sensor. It is not a breakdown, but rather a communication check where the system is waiting for data to ensure your heating schedule is accurate. If the message stays on screen, it usually means the external sensor is having trouble picking up a clear signal from the transmitter.
This code may be more noticeable during the clocks changing in spring and autumn when the boiler actively seeks a signal to update its time.
DIY fix possible
Wait up to 10 minutes for the signal search to completeEnsure there are no large metal objects blocking the signal to the outdoor sensorIf the code persists, the boiler will simply operate using its internal clock
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Gas Safe Register →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.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026