L1 High

Pro L1 Fault Code

The Pro L1 fault code means: your boiler has switched itself off because the water inside it has become too hot, likely due to a circulation problem.. It appears on 1 Pro 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 OVERHEAT LOCKOUT
Typical repair cost £120-£350 (including labour)

DIY fix possible

Check that all radiator valves are fully openEnsure the boiler pressure gauge is between 1 and 1.5 barPress and hold the Reset button for 2 seconds

Find a Gas Safe engineer

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

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Boiler cover options

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Pro models that show the L1 fault code

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

Source

Pro L1 fault code information on this page is sourced and cross-checked against the official Pro service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026