How much does it cost to fix boiler error code E784?

Typical repair cost for the boiler E784 fault is £150-300 including labour.

E784 High

Boiler Fault Code E784

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 7 July 2026

Fault code E784 appears across 1 boiler brand and 3 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler has lost the connection to the system that tells it which parts of your home need heating, causing the unit to stop operating correctly. (As reported on Navien NCM 1130HH)

Pick your boiler brand for the E784 fix

Navien E784 3 models

Source

Information for the E784 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