BoilerFaultCodesVaillant D.12 Fault Code
Also written as D12 — same fault.
The Vaillant D.12 fault code means: your boiler is having trouble communicating with the pump that heats your hot water cylinder, meaning you may experience a lack of hot water.. It appears on 7 Vaillant models in our database.
Technical description: Cylinder charging pump via accessory module
What causes the Vaillant D.12 fault?
This fault occurs when the boiler's main control board loses its connection with the external pump responsible for heating your hot water tank. It is usually caused by a loose electrical connection, a failing communication module, or the pump itself developing an internal electronic fault that prevents it from responding to the boiler.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the D.12 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.12 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