C 20 High

Atag C 20 Fault Code

Also written as C20 — same fault.

The Atag C 20 fault code means: your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, which has caused it to stop running for safety.. It appears on 2 Atag 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: Flow sensor error (e.g. open, short circuit, outside of range)
Typical repair cost £100-£190 (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 20 fault code

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

Source

Atag C 20 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 7 July 2026