BoilerFaultCodesVaillant F23 Fault Code
The Vaillant F23 fault code means: your boiler has temporarily 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 protection activated
DIY fix possible
Locate the filling loop underneath your boiler, which usually consists of a flexible silver hose with one or two small valves.Slowly open the valves to let mains water into the heating system, keeping an eye on the pressure gauge.Stop filling once the needle on the pressure gauge reaches the green zone (ideally between 1.0 and 1.5 bar) and firmly close both valves.
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Vaillant models that show the F23 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F23 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