How much does it cost to fix boiler error code E121?

Typical repair cost for the boiler E121 fault is £150-250 including labour.

E121 High

Boiler Fault Code E121

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 27 April 2026

Fault code E121 appears across 1 boiler brand and 4 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler has stopped because its internal sensors have detected that the water inside the system is not circulating correctly. (As reported on Navien NCB 28 HE)

Pick your boiler brand for the E121 fix

Navien E121 4 models

Source

Information for the E121 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