Navien tankless water heater code E786: Panel connection fault

Tankless water heater, United States

The heater reported abnormal operation of the connection to its control panel.

This needs a licensed gas or HVAC technician. There is nothing safe to check yourself. Leave the covers on and book a professional.

What causes it

A fault in the panel connection that needs a technician. Navien lists no owner fix for this code.

Book a licensed gas or HVAC technician

Have the model number, serial number, purchase date, where the heater is installed and the error code ready.
Call Navien Technical Support on 1-800-519-8794, or contact the company that installed the heater.
For a repair under warranty, the installer or service provider must contact Navien before starting the work.

Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
Note the code, how often it appears, and the model and serial numbers from the rating label.
Book a licensed gas or HVAC technician and give them those details.

7 models in 2 ranges

Range	Models
NPN-E	NPN-120E, NPN-160E, NPN-180E, NPN-199E
NPN-U	NPN-160U, NPN-180U, NPN-199U

Checked against the manufacturer's manual on 15 October 2026.

Code	Meaning	What you can do
E517	DIP switch setting error	Call a pro
E593	Front panel button fault	Call a pro
E594	Memory chip (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	Call a pro
E760	Flushing or service reminder	Call a pro
E762	Vent pipe not fully inserted	Call a pro
E773	Wrong gas type detected or gas connection fault (depends on model)	Call a pro
E782	Main panel communication fault	Call a pro
E785	Flow switch or sensor fault	Call a pro
E788	Gas type setting mismatch	Call a pro