How much does it cost to fix Atag error code C21?

Typical repair cost for the Atag C21 fault is £150-250 including labour.

C21 High

Atag C21 Fault Code

Q: What does the Atag C21 fault code mean?

Your boiler has detected a communication issue with the sensor that monitors exhaust gases and has safely shut down to prevent potential overheating or unsafe operation.

The Atag C21 fault code means: your boiler has detected a communication issue with the sensor that monitors exhaust gases and has safely shut down to prevent potential overheating or unsafe operation.. It appears on 1 Atag model in our database.

Technical description: Flue sensor fault

Typical repair cost £150-250 (including labour)

Engineer required

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

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Atag models that show the C21 fault code

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

Premier Repair: £150-250

Source

Atag C21 fault code information on this page is sourced and cross-checked against the official Atag service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026