BoilerFaultCodesEvo H9/L9 Fault Code
Also written as H9L9 — same fault.
The Evo H9/L9 fault code means: your boiler has shut down because the internal water temperature has become too hot, causing it to lock out for safety.. It appears on 1 Evo model in our database.
Technical description: Heat Exchanger Overheat
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barTry resetting the boiler once the unit has cooled down
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Evo models that show the H9/L9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Evo H9/L9 fault code information on this page is sourced and cross-checked against the official Evo service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026