Navien tankless water heater code E439: Flow sensor fault

Tankless water heater, Canada and United States

The sensor that measures water flow through the heater is not operating normally.

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

What causes it

The flow sensor or its wiring has failed. 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.

21 models in 6 ranges

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

Code	Meaning	What you can do
E004	False flame detected	Safe checks first, then a pro
E407	Hot water outlet temperature sensor fault	Call a pro
E421	Cold water inlet temperature sensor 1 fault	Call a pro
E432	Cold water inlet temperature sensor 2 fault	Call a pro
E434	Water adjustment valve fault	Call a pro
E438	Circulation pump fault	Safe checks first, then 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
E785	Flow switch or sensor fault	Call a pro