12 Medium

Atag 12 Fault Code

The Atag 12 fault code means: your boiler has detected that the water inside the system is getting too hot too quickly while trying to heat your tap water, so it has temporarily paused hot water production to prevent damage.. It appears on 6 Atag models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Maximum (cid:1)T of flow and return sensor in domestic hot water has repeatedly been exceeded. During the block normal operation of the central heating installation is possible. During the block the pump continues to operate at minimum capacity.
Typical repair cost £150-300 (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 12 fault code

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

Source

Atag 12 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