How much does it cost to fix Atag error code C18?

Typical repair cost for the Atag C18 fault is £150-250 including labour.

C18 High

Atag C18 Fault Code

Q: What does the Atag C18 fault code mean?

Your boiler is struggling to detect the flow of water because a sensor is failing, which means it has stopped heating your home or water to prevent internal damage.

The Atag C18 fault code means: your boiler is struggling to detect the flow of water because a sensor is failing, which means it has stopped heating your home or water to prevent internal damage.. It appears on 1 Atag model in our database.

Technical description: Flow 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 C18 fault code

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

Premier Repair: £150-250

Source

Atag C18 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