d4 Low

Boiler Fault Code d4

Also written as D4 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code d4 appears across 3 boiler brands and 16 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 taking a temporary break to protect its internal components from overheating, which is a normal safety feature after the system has reached its operating limit. (As reported on Ferroli Bluehelix Tech RRT 24C)

Pick your boiler brand for the d4 fix

Alpha d4 9 models Ferroli d4 3 models Worcester Bosch d4 4 models

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 7 July 2026