BoilerFaultCodesVaillant E12 Fault Code
The Vaillant E12 fault code means: your boiler has stopped working because the water pressure inside the system has dropped too low.. It appears on 2 Vaillant models in our database.
Technical description: Water deficiency error – Appliance blocked
DIY fix possible
Locate the filling loop beneath your boiler.Open the valves gently to let water into the system until the pressure gauge reads between 1.0 and 1.5 bar.Close both valves firmly and check the pressure.
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Vaillant models that show the E12 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant E12 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 31 August 2026