BoilerFaultCodes 594 High
Boiler Fault Code 594
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 594 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 detected an internal communication problem with the air pressure sensor, which is preventing it from safely starting up. (As reported on Navien NCB 28 HE)
Pick your boiler brand for the 594 fix
Source
Information for the 594 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