Product troubleshooting

Navien NFB-C 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.

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Error-code table

Search codes for NFB-C Series

Search error codes

Code	Meaning	System area	Safe first step	Action
E001	Heat exchanger overheat	Equipment control	Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service.	View details
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
E030	Exhaust overheat	Combustion / venting	Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service.	View details
E031	Burner overheat	Combustion / venting	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
E047	Exhaust thermostat or thermistor open/short	Combustion / venting	Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service.	View details
E060	Abnormal operation: dual venturi	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
E205	Heating supply 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
E218	Heating return 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
E278	External supply sensor not 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
E279	External return sensor not 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
E291	Return temperature exceeds supply temperature	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
E302	Low water pressure	Water flow / hydronics	Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service.	View details
E352	High water pressure	Water flow / hydronics	Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service.	View details
E353	Water pressure sensor issue	Temperature / sensor	Record the Navien code, avoid repeated reset attempts, and schedule GreenFlow service.	View details
E480	Domestic hot water tank temperature sensor not 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
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
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
E765	Abnormal external pump operation	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
E774	Manual reset limit	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
E775	Auto reset limit	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
E777	Low water cutoff activated	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
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
E784	Abnormal operation: zone controller 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

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.

Safe visible checks

Thermostat display and settings
Filter condition and return-air path
Visible outdoor-unit obstruction
Photos of the code, unit, and nameplate

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

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