d.66 Low

Glow-worm d.66 Fault Code

Also written as D66 — same fault.

The Glow-worm d.66 fault code means: this is not an error code, but rather an indicator that your boiler is currently maintaining a small amount of hot water in its internal heat exchanger to ensure you get hot water at your taps faster.. It appears on 4 Glow-worm models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Activation of the warm start function for hot water

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Glow-worm models that show the d.66 fault code

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

Source

Glow-worm d.66 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.

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