How much does it cost to fix boiler error code 421?

Typical repair cost for the boiler 421 fault is £180-300 including labour.

421 High

Boiler Fault Code 421

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 27 April 2026

Fault code 421 appears across 1 boiler brand and 6 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 problem with the internal flow sensor, which means it cannot accurately measure the movement of water to heat your home. (As reported on Navien NCB CE)

Pick your boiler brand for the 421 fix

Navien 421 6 models

Source

Information for the 421 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