Safe visible checks
- Thermostat display and settings
- Filter condition and return-air path
- Visible outdoor-unit obstruction
- Photos of the code, unit, and nameplate
Product troubleshooting
Navien NPN-U Series troubleshooting and error codes
Use this table to identify what the visible code may mean and what information to share when scheduling service. Keep checks simple and visible; do not open electrical, refrigerant, gas, or sealed equipment compartments.
Error-code table
Search codes for NPN-U Series
| Code | Meaning | System area | Safe first step | Action |
|---|---|---|---|---|
| E003 | Ignition failure | Combustion / venting | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E004 | False flame detection | Combustion / venting | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E012 | Flame loss | Combustion / venting | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E016 | Heat exchanger overheat | Equipment control | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E046 | Abnormal operation: heat exchanger limit or thermistor | Temperature / sensor | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E056 | Abnormal operation: modulating gas valve | Combustion / venting | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E109 | Abnormal operation: fan motor | Airflow / fan | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E110 | Abnormal air pressure or exhaust blockage | Combustion / venting | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E127 | Abnormal operation: air pressure sensor or air volume control | Combustion / venting | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E157 | Abnormal operation: air volume control | Airflow / fan | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E170 | Antifreeze heater open or short | Equipment control | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E407 | Domestic hot water outlet thermistor open or short | Temperature / sensor | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E421 | Cold water inlet thermistor open or short | Temperature / sensor | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E434 | Abnormal operation: water adjustment valve | Water flow / hydronics | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E438 | Abnormal operation: circulation pump | Water flow / hydronics | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E439 | Abnormal operation: flow sensor or no domestic water flow detected | Temperature / sensor | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E441 | Domestic hot water inlet/outlet 2 sensor open or short | Temperature / sensor | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E445 | Abnormal operation: bypass mixing valve | Water flow / hydronics | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E515 | Abnormal operation: PCB | Controls / communication | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E517 | Abnormal operation: DIP switch setting | Controls / communication | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E593 | Abnormal operation: front panel key | Controls / communication | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E594 | Abnormal operation: EEPROM | Controls / communication | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E615 | Abnormal operation: input and memory | Controls / communication | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E736 | Abnormal operation: cascade communication | Controls / communication | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E740 | Abnormal operation: outdoor temperature sensor | Temperature / sensor | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E760 | Flushing or service alarm | Equipment control | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E762 | Vent pipe not properly inserted | Combustion / venting | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E773 | Improper gas type or gas connection detected | Combustion / venting | Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service. | View details |
| E782 | Abnormal operation: front/main panel communication | Controls / communication | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E785 | Abnormal operation: flow switch or sensor | Temperature / sensor | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
| E786 | Improper front panel or panel connection | Controls / communication | Record the Navien code, check visible intake, exhaust, filter, thermostat, controller, and clearance conditions without opening panels, and schedule service if the condition remains. | View details |
No matching error codes found.
Safety note
What is safe to check before service
These pages are meant to help document the issue, not replace diagnosis. GreenFlow should handle pressure-switch, electrical, refrigerant, lockout, recurring, and warranty-related conditions.
Stop and call
- Repeated lockouts or recurring faults
- Pressure-switch or sensor faults
- Electrical odor, unusual noise, or ice buildup
- Any sealed cabinet, refrigerant, gas, or warranty issue
Brands We Service
No matching featured brand found.
Popular cities
Service Areas
No matching service area found
Try another nearby city or contact GreenFlow to confirm service availability.
Our Happy Customers
Dev W.
I had a breakdown of my cooling system and Hanna at Green Flow responded to my request right away. They scheduled me same day. The team at Green Flow was easy to communicate with over phone and text. They kept me updated on delays on arrival of the...
Anna F.
They showed up within an hour of me making the call on a Sunday. They fixed our system and taught us how to replace the fuse in the future. We would definitely call again.
John F.
I needed 2 new AC units and decided to go with high efficiency heat pumps. I chose Greenflow because of the equipment they offered, and the price (2nd lowest quote and much cheaper than the average of all quote we received). Their technicians were very detail oriented and...
Jahangir R.
They were very organized to communicate and be on time to come to look at my air condition to fix it.