BoilerFaultCodes E777 High
Boiler Fault Code E777
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code E777 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 detected that a safety device installed outside the main unit is reporting a dangerously low water level, preventing the boiler from starting to protect it from damage. (As reported on Navien NCB-28LDWE)
Pick your boiler brand for the E777 fix
Source
Information for the E777 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