Navien tankless water heater code E594: Memory chip (EEPROM) fault

Tankless water heater, Canada and United States

The heater detected a fault in its EEPROM, the memory chip that holds its settings.

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

What causes it

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

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.

14 models in 4 ranges

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

Code	Meaning	What you can do
E439	Flow sensor fault	Call a pro
E441	Hot water inlet temperature sensor 2 fault	Call a pro
E445	Bypass mixing valve fault	Call a pro
E515	Circuit board fault	Safe checks first, then a pro
E517	DIP switch setting error	Call a pro
E593	Front panel button 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