i4 Medium

Boiler Fault Code i4

Also written as I4 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code i4 appears across 2 boiler brands and 2 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 a communication error with an external temperature sensor, which may prevent it from operating correctly or heating your home. (As reported on Sime Metropolis DGT 25 BF)

Pick your boiler brand for the i4 fix

Sime i4 1 model Worcester Bosch i4 1 model

Source

Information for the i4 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