Navien tankless water heater code E788: Gas type setting mismatch

Tankless water heater, Canada and United States

The gas type set by the DIP switch does not match the gas type set on the front panel.

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

This involves the gas supply or burner. Only a licensed technician should open the unit or work on gas parts.

What causes it

The gas type DIP switch and the front panel gas setting disagree, for example after a gas conversion. A technician confirms the gas supplied and runs the panel's gas type check to match them.

Book a licensed gas 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 technician and give them those details.

7 models in 2 ranges

Range	Models
NPE-A2	NPE-180A2, NPE-210A2, NPE-240A2
NPE-S2	NPE-180S2, NPE-210S2, NPE-240S2, NPE-150S2

Checked against the manufacturer's manual on 9 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
E786	Panel connection fault	Call a pro