BoilerFaultCodes E740 Low
Boiler Fault Code E740
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code E740 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 has lost the connection to the thermometer installed outside your home, so it cannot automatically adjust your heating based on the current weather. (As reported on Navien NCB CE)
Pick your boiler brand for the E740 fix
Source
Information for the E740 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