How much does it cost to fix Atag error code C 40?

Typical repair cost for the Atag C 40 fault is £150-250 including labour.

C 40 High

Atag C 40 Fault Code

Q: What does the Atag C 40 fault code mean?

Your boiler is struggling to accurately measure the temperature of the water coming back into the system, so it has safely shut down to prevent overheating.

Also written as C40 — same fault.

The Atag C 40 fault code means: your boiler is struggling to accurately measure the temperature of the water coming back into the system, so it has safely shut down to prevent overheating.. It appears on 4 Atag models in our database.

Technical description: Return sensor error (e.g. open, short circuit, outside of range)

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 C 40 fault code

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

Source

Atag C 40 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