E21 High

Ariston E21 Fault Code

The Ariston E21 fault code means: your boiler has detected that the electricity supply wires have been connected in the wrong order, preventing it from operating safely.. It appears on 4 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: Error in the electrical connection (live and neutral crossed)
Typical repair cost £90-150 (including labour)

Engineer required

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

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like E21: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

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 7 July 2026