BoilerFaultCodesVaillant D.98 Fault Code
Also written as D98 — same fault.
The Vaillant D.98 fault code means: your boiler's communication system between the main computer and the burner control is experiencing an internal error, which has caused the unit to shut down for safety reasons.. It appears on 2 Vaillant models in our database.
Technical description: Telephone:Competentperson
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.98 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.98 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 7 July 2026