BoilerFaultCodesAriston A21 Fault Code
The Ariston A21 fault code means: your boiler has stopped heating because it is struggling to move water through the system, often because the water pressure is too low or the system is blocked.. It appears on 4 Ariston models in our database.
Technical description: Shutdown Due to Insufficient Water Circulation
DIY fix possible
Check your boiler pressure gauge; if it is below 1 bar, use the filling loop to top it up to 1.5 bar.Ensure all radiator valves are fully open to allow water to flow freely.Restart the boiler using the reset button once the pressure is correct.
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Ariston models that show the A21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ariston A21 fault code information on this page is sourced and cross-checked against the official Ariston service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026