18 High

Atag 18 Fault Code

The Atag 18 fault code means: your boiler has shut down because the water inside the unit has reached a temperature that is too hot to operate safely.. It appears on 1 Atag model 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 temperature too high
Typical repair cost £120-350 (including labour)

DIY fix possible

Check that all radiator valves (TRVs) are turned to the highest settingEnsure the boiler pressure is between 1.0 and 1.5 barWait 30 minutes for the unit to cool down and then press the Reset button

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Atag models that show the 18 fault code

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

Source

Atag 18 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