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
The same code number can mean something different on another series — make sure you're reading the page for your exact unit:
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: