BoilerFaultCodesVaillant D.26 Fault Code
Also written as D26 — same fault.
The Vaillant D.26 fault code means: your boiler's internal control board is unable to communicate with an optional secondary accessory or pump, causing the system to stop running as a safety precaution.. It appears on 4 Vaillant models in our database.
Technical description: Additional relay control ecoTec
What causes the Vaillant D.26 fault?
This fault occurs when the boiler's main control centre loses its connection to an extra electrical component, such as an external pump or specialized heating control. The most common reason is a loose connection in the internal wiring or a component failure within the electronic control board. Because the boiler can no longer coordinate with these essential external parts, it shuts down to prevent any mechanical damage.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Vaillant models that show the D.26 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.26 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