BoilerFaultCodes d.128 Low
Boiler Fault Code d.128
Also written as D128 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code d.128 appears across 2 boiler brands and 4 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is reporting a minor configuration warning indicating that the target temperature for your heating system is currently set lower than the boiler's internal safety parameters. (As reported on Glow-worm Sustain 12s)
Pick your boiler brand for the d.128 fix
Source
Information for the d.128 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