Navien NPE-A / NPE-S Code E407
Hot water outlet thermistor: open or short circuit. The sensor reading the temperature of water leaving the unit is either open circuit or shorted — an electrical fault in the sensor or its wiring rather than a reading that is merely wrong. Navien's instruction is to contact technical support.
Part replacementThe verdict
Replace a part
An inexpensive sensor with a service call around it. If it comes out coated in scale, the sensor was the symptom and the water is the cause.
Typical cost: $150–$450
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the NPE-A / NPE-S — not averaged across the brand.
Source: Navien NPE Operation Manual (NPE-A / NPE-S) (p. 16), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note when the unit was last flushed — thermistors in the water path are where scale shows up first.
- Book a service call; the unit has to be isolated to reach the sensor.
Code E407 on other Navien series
Other Navien series report code E407 under the same fault name, but each manual is transcribed separately and they do not read alike:
Don't confuse this with
3 other Navien NPE-A / NPE-S codes point at a thermistor too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: