D1 High

Gledhill D1 Fault Code

The Gledhill D1 fault code means: your boiler has experienced a lockout, meaning it has switched itself off for safety after failing to fire correctly.. It appears on 1 Gledhill 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: Flashing - Lockout error / Press reset button 'S1' to clear
Typical repair cost £90-250 (including labour)

DIY fix possible

Locate the reset button (labeled S1) on the control panel.Press and hold the reset button for a few seconds to see if the boiler restarts.If the boiler locks out again immediately, do not keep resetting it and call a professional.

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Gledhill models that show the D1 fault code

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

Source

Gledhill D1 fault code information on this page is sourced and cross-checked against the official Gledhill service / installation manuals for each affected model.

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