How much does it cost to fix boiler error code E421?

Typical repair cost for the boiler E421 fault is £120-£220 including labour.

E421 High

Boiler Fault Code E421

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 27 April 2026

Fault code E421 appears across 1 boiler brand and 7 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 28 HE)

Pick your boiler brand for the E421 fix

Navien E421 7 models

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 27 April 2026