Navien tankless water heater code E773: Wrong gas type detected or gas connection fault

Tankless water heater, Canada and United States

This code means one of two things, depending on your model. Find your model family below, then read the section for it. The model number is on the rating label on the side of the unit.

Find your model family

If you have	It means
NPE-A2, NPE-S2	Wrong gas type detected
NPN-E, NPN-U	Gas connection fault

Wrong gas type detected

On NPE-A2, NPE-S2.

The heater detected that the gas connected to it does not match the gas type set at installation.

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 supplied does not match the heater's gas type setting (DIP switch 1 off for natural gas, on for propane).

Book a licensed gas technician
Measuring tolerances can trigger the code even when the gas and the setting match. If it persists once a technician has confirmed the setting, Navien technical support needs to look at it.

Book a licensed gas technician

How to read this code on your unit

Booking a licensed gas technician

Contact Navien before booking a repairYou can do this

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.

Book a licensed gas technicianlicensed gas technician

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.

Models that show this code

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

Where this comes from

Checked against the manufacturer's manual on 9 October 2026.

Gas connection fault

On NPN-E, NPN-U.

The heater reported abnormal operation of its gas connection.

This needs a licensed gas or HVAC 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

A problem with the heater's gas connection that a technician needs to find. Navien lists no owner fix for this code.

Book a licensed gas or HVAC technician

How to read this code on your unit

Booking a licensed gas or HVAC technician

Contact Navien before booking a repairYou can do this

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.

Book a licensed gas or HVAC technicianlicensed gas or HVAC technician

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.

Models that show this code

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

Where this comes from

User's Information Manual, NPN non-condensing tankless water heaters (20045995B) , page 23

Checked against the manufacturer's manual on 9 October 2026.

Other Navien tankless water heater codes

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
E782	Main panel communication fault	Call a pro
E785	Flow switch or sensor fault	Call a pro
E786	Panel connection fault	Call a pro
E788	Gas type setting mismatch	Call a pro

See all configuration error codes on tankless water heaters across brands.

Report a mistake on this page