Navien E407: hot water outlet sensor fault

A tankless heater adjusts its flame based on how hot the water leaving it is. The hot water outlet thermistor provides that reading. E407 means the reading is electrically invalid, so the heater cannot control the temperature safely and stops.

The meaning below comes from Navien's manuals for its NPE condensing tankless water heaters. Many codes are shared with Navien combi boilers, but check the manual for your model if something does not match.

For this code, Navien's user manual directs owners to contact Navien technical support (1-800-519-8794) or a licensed installer.

Common causes

• A loose or corroded connector at the sensor
• A damaged wire between the sensor and the control board
• A sensor that has failed, sometimes after scale or water exposure

The repair

A technician checks the connector and wiring, then measures the sensor. If it reads open or shorted, it is replaced. The part is inexpensive; the work involves shutting off the water and working inside the cabinet.

How to reset a Navien

1. Close all hot water taps.
2. Press the power button on the front panel to turn the unit off, wait about a minute, then turn it back on.
3. Open a hot tap and watch whether the code returns.

If E407 returns right away, the sensor or wiring still has a fault.

See every code on our Navien error codes page.