Navien NPE-2 error codes
The current NPE generation. A2 models add a built-in recirculation pump, a recirculation inlet filter and a pump drain plug that the S2 models do not have — which is why several codes here say "in case of installed AHU" or name a pump your unit may not own. Its code table is longer than the original NPE's: E031, E594, E762, E773 and E788 exist only on this generation, and two of those — wrong gas type, and a vent pipe that was never fully seated — are faults the older units simply never told anyone about.
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 |
| E031 | Overheating of burner | Call a pro |
| E046 | Limit control fault | Call a pro |
| E047 | Exhaust thermostat: open or short circuit | 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 |
| E594 | 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 | Part replacement |
| E760 | Flushing / service alarm | DIY fix |
| E762 | Vent pipe not properly inserted | Call a pro |
| E773 | Using the wrong gas | Call a pro |
| E782 | Main panel communication fault | Call a pro |
| E785 | Flow switch or flow sensor fault | Part replacement |
| E788 | Gas type setting does not match the panel | Call a pro |
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-2, check yours: