BoilerFaultCodesVaillant I.020 Fault Code
Also written as I020 — same fault.
The Vaillant I.020 fault code means: your boiler has detected that the water pressure in your heating system has dropped too low, so it has temporarily stopped working to protect itself.. It appears on 3 Vaillant models in our database.
Technical description: Filling pressure of the heating installation is low. The water pressure in the heating system is at the lower limit.
DIY fix possible
Locate the external filling loop, which is usually a silver flexible hose connected to two valves under the boiler.Slowly open both valves to allow cold mains water to enter the system until the gauge on the front of the boiler reads between 1.0 and 1.5 bar.Close both valves tightly and ensure the pressure remains stable.
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Vaillant models that show the I.020 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant I.020 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