A21 High

Ariston A21 Fault Code

The Ariston A21 fault code means: your boiler has detected that water is not circulating correctly through the system, preventing it from firing up to provide heat or hot water.. It appears on 3 Ariston models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: The flow switch does not close
Typical repair cost £120-£280 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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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 27 April 2026