BoilerFaultCodes d.99 Low
Boiler Fault Code d.99
Also written as D99 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code d.99 appears across 1 boiler brand and 1 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is currently displaying a general information status code rather than a specific fault, often indicating it is cycling through its internal diagnostic test mode. (As reported on Glow-worm Ultracom hxi)
Pick your boiler brand for the d.99 fix
Source
Information for the d.99 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