Navien NPE-2 Code E407
Hot water outlet thermistor: open or short circuit. The sensor reading the temperature of water leaving the heater has an open or shorted circuit, so the unit has lost its main feedback on how hot the water actually is. Navien's instruction is to contact technical support.
Part replacementThe verdict
Replace a part
A cheap sensor with an expensive-sounding name. The part is one of the least costly in the unit; the bill is the visit, the drain-down, and the leak check afterwards. On hard water, scale on the sensor is worth asking about before it is replaced.
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-2 — not averaged across the brand.
Source: Navien NPE-2 User's Information Manual (p. 17), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note whether the water temperature has been wandering at the tap — useful detail for the technician.
- Note when the unit was last flushed and whether your water is hard.
- Contact your installer or Navien technical support.
Frequently asked questions
What does "open or short circuit" mean here?
A thermistor's resistance should sit within a known band. Open means the circuit is broken — a cut wire, a disconnected plug, a failed sensor. Short means it reads near zero. Either way the board knows the reading is not a real temperature.
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-2 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: