BoilerFaultCodes E421 High
Boiler Fault Code E421
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code E421 appears across 1 boiler brand and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is having trouble measuring incoming water temperature due to a faulty sensor, which is currently preventing it from heating your water correctly. (As reported on Navien NCB-28LDWE)
Pick your boiler brand for the E421 fix
Source
Information for the E421 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