BoilerFaultCodesVaillant D.90 Fault Code
Also written as D90 — same fault.
The Vaillant D.90 fault code means: your boiler is currently waiting for a signal from an external smart control or thermostat, meaning it is in a standby or communication phase rather than a fault.. It appears on 5 Vaillant models in our database.
Technical description: Digital regulator status
What causes the Vaillant D.90 fault?
This code appears when the boiler is waiting for a command from your external thermostat or heating controls to start heating. It usually indicates that the target temperature has been reached or that there is a temporary communication gap between the smart controller and the boiler's internal computer. The boiler isn't broken; it is simply sitting in a 'ready' state until your sensors tell it that your home needs more heat.
DIY fix possible
Check that your room thermostat is turned up higher than the current room temperatureEnsure your external heating programmer is set to 'ON' or 'AUTO'Check the batteries in your wireless thermostat or smart controller
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Gas Safe Register →Vaillant models that show the D.90 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.90 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