Navien NPE-A / NPE-S error codes
The first-generation NPE, and still the one in most American basements that had a Navien fitted before the NPE-2 arrived. Telling them apart takes one glance at the model number: this generation is NPE-240A, the next is NPE-240A2 — the trailing "2" is the whole difference in the badge and it changes which code table applies. A models carry a built-in recirculation pump; S models do not. Its table is five codes shorter than the NPE-2's: E031, E594, E762, E773 and E788 do not exist here, which means this generation never told anyone it had been fed the wrong gas type or that a vent pipe was not fully seated. Four codes it does share are worded differently, and on two of them — E016 and E030 — Navien gives owners a step it later dropped: turn the system off for at least 30 minutes before anything else.
Not your models? Check the model-number decoder before you read the table below — the code numbers repeat across series and the meanings do not.
| Code | Error | Verdict |
|---|---|---|
| E003 | Ignition failure | Call a pro |
| E004 | False flame detection | Call a pro |
| E012 | Flame loss | Call a pro |
| E016 | Overheating of heat exchanger | Call a pro |
| E030 | Exhaust overheating | Call a pro |
| E046 | Limit control fault | Call a pro |
| E047 | Exhaust thermostat fault | Call a pro |
| E060 | Dual venturi fault | Call a pro |
| E109 | Fan motor fault | Call a pro |
| E110 | Abnormal air pressure | Call a pro |
| E407 | Hot water outlet thermistor: open or short circuit | Part replacement |
| E421 | Cold water inlet thermistor 1: open or short circuit | Part replacement |
| E432 | Cold water inlet thermistor 2: open or short circuit | Part replacement |
| E434 | Water adjustment valve fault | Part replacement |
| E438 | Circulation pump fault | Part replacement |
| E439 | Flow sensor fault | Part replacement |
| E441 | Hot water inlet thermistor 2: open or short circuit | Part replacement |
| E445 | Bypass mixing valve fault | Part replacement |
| E515 | PCB fault | Call a pro |
| E517 | DIP switch setting fault | Call a pro |
| E593 | Front panel key fault | Part replacement |
| E615 | Input and memory fault | Call a pro |
| E736 | Cascade communication fault | Call a pro |
| E740 | Outdoor temperature sensor fault | Part replacement |
| E760 | Flushing / service alarm | DIY fix |
| E782 | Main panel communication fault | Call a pro |
| E785 | Flow switch or flow sensor fault | Part replacement |
Every entry above is transcribed from the manufacturer's diagnostic documentation for this exact series, with the source cited on each code page.
Other Navien series
Different series, different code table — if your unit isn't a NPE-A / NPE-S, check yours: