BoilerFaultCodes D4 High
Boiler Fault Code D4
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code D4 appears across 1 boiler brand and 3 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler has detected that the water temperature is changing too quickly or unevenly between different parts of the system, so it has safely shut itself down as a precaution. (As reported on Worcester Bosch Greenstar CDi Compact)
Pick your boiler brand for the D4 fix
Source
Information for the D4 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 27 April 2026