d / U Low

Boiler Fault Code d / U

Also written as DU — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code d / U 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 undergoing a routine self-test of its internal valve to ensure it doesn't get stuck and is not actually a fault code. (As reported on Pro ProCombi Exclusive 24)

Pick your boiler brand for the d / U fix

Pro d / U 1 model

Source

Information for the d / U 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