P.12 Low

Vaillant P.12 Fault Code

Also written as P12 — same fault.

The Vaillant P.12 fault code means: your boiler is undergoing a standard automatic self-test procedure during startup and is temporarily pausing to check its internal systems.. It appears on 2 Vaillant models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Not applicable - for GB

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 P.12 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant P.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