d.66 Low

Boiler Fault Code d.66

Also written as D66 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code d.66 appears across 2 boiler brands and 13 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning 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. (As reported on Glow-worm Ultimate 3 25R)

Pick your boiler brand for the d.66 fix

Glow-worm d.66 4 models Vaillant d.66 9 models

Source

Information for the d.66 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →

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