Navien NPN-U error codes

Every checked code for the Navien NPN-U range, with what it means and whether you can fix it yourself.

Error codes

Code	Meaning	What you can do
E003	Ignition failure	Safe checks first, then a pro
E004	False flame detected	Safe checks first, then a pro
E012	Flame lost	Safe checks first, then a pro
E016	Heat exchanger overheated	Safe checks first, then a pro
E046	Limit control fault	Call a pro
E056	Modulating gas valve fault	Call a pro
E109	Fan motor fault	Safe checks first, then a pro
E110	Abnormal air pressure (blocked vent or intake filter)	Safe checks first, then a pro
E127	Air volume sensor fault	Call a pro
E157	Air volume control fault	Call a pro
E170	Freeze protection heater fault	Call a pro
E407	Hot water outlet temperature sensor fault	Call a pro
E421	Cold water inlet temperature sensor 1 fault	Call a pro
E434	Water adjustment valve fault	Call a pro
E438	Circulation pump fault	Safe checks first, then a pro
E439	Flow sensor fault	Call a pro
E441	Hot water inlet temperature sensor 2 fault	Call a pro
E445	Bypass mixing valve fault	Call a pro
E515	Circuit board fault	Safe checks first, then a pro
E517	DIP switch setting error	Call a pro
E593	Front panel button fault	Call a pro
E594	Memory chip (EEPROM) fault	Call a pro
E615	Input and memory fault	Call a pro
E736	Cascade communication fault	Call a pro
E740	Outdoor temperature sensor fault	Call a pro
E760	Flushing or service reminder	Call a pro
E762	Vent pipe not fully inserted	Call a pro
E773	Wrong gas type detected or gas connection fault (depends on model)	Call a pro
E782	Main panel communication fault	Call a pro
E785	Flow switch or sensor fault	Call a pro
E786	Panel connection fault	Call a pro