E21 Emergency

Ariston E21 Fault Code

The Ariston E21 fault code means: your boiler has detected that the electrical wiring coming into the unit is connected incorrectly, which is a safety conflict between the live and neutral wires.. It appears on 4 Ariston models in our database.

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

Technical description: Error in the electrical connection (live and neutral crossed)
Typical repair cost £80-160 (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 E21 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Ariston E21 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