Navien NFC Series User's Information Manual page 23

Error Code Reason
E615 Abnormal operation: input and memory
E736 Abnormal operation: cascade communication
Abnormal operation: outdoor temperature sensor (appears only when the
E740
outdoor reset curve is enabled).
E761 Abnormal Operation: Pre-Heat Aquastat
E777 LWCO (service code: 7700) (external device)
E782 Abnormal operation: main panel communication
E784 Abnormal operation: zone controller communication
If these remedies do not resolve the problem, contact Technical Support at 1-800-519-8794.
22 Troubleshooting
Self-diagnostic/Action
Manual RESET
Auto RESET
Alarm
Alarm
Alarm
Alarm
Auto RESET
Alarm
Alarm