Navien tankless water heater code E127: Air volume sensor fault

Tankless water heater, United States

The air volume sensor is reading as an open or short circuit, is stuck at its default value, or is not responding.

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 flue or exhaust. A blocked or leaking flue can let carbon monoxide into the home.

What causes it

The air volume sensor or its wiring has failed. 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 9 October 2026.

Code	Meaning	What you can do
E046	Limit control fault	Call a pro
E047	Exhaust temperature sensor fault	Call a pro
E056	Modulating gas valve fault	Call a pro
E060	Dual venturi fault	Call a pro
E109	Fan motor fault	Safe checks first, then a pro
E110	Abnormal air pressure (blocked vent or intake filter)	Safe checks first, then a pro
E157	Air volume control fault	Call a pro
E170	Freeze protection heater fault	Call 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
E762	Vent pipe not fully inserted	Call a pro