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 temporary waiting process while it determines if the water pressure is high enough to run safely.. It appears on 1 Vaillant model 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
Typical repair cost £0-60 (including labour)

DIY fix possible

Check the pressure gauge on the front of the boiler.If the pressure is below 1.0 bar, use the filling loop handles to top it up to 1.5 bar.Restart the boiler once the pressure is correct.

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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 7 July 2026