Navien tankless water heater code E407
Hot water outlet thermistor: open or short circuit. Code E407 means the same thing on all 2 Navien series documented here — but the fix, and what it costs, is still worth reading for your exact unit.
What code E407 means on each Navien series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| NPE-2 | Hot water outlet thermistor: open or short circuit | Part replacement | $150–$450 |
| NPE-A / NPE-S | Hot water outlet thermistor: open or short circuit | Part replacement | $150–$450 |
Each row opens the full page for that generation. We hold manuals for NPE-2 and NPE-A / NPE-S, and each one gets its own reading of code E407 — what Navien documents, what you can safely check yourself, and what the repair should cost.
What each manual actually says
The table above compares 2 series in four columns, which is enough to route you and not enough to explain anything. This is the Navien wording behind each row, one entry per manual, so you can see where the 2 readings of code E407 agree and where they part company.
NPE-2 — 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.
NPE-A / NPE-S — 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.
Does it matter which Navien you have?
NPE-2 and NPE-A / NPE-S all route code E407 the same way: a replacement part, at $150–$450. That makes it one of the Navien numbers you can act on before working out which unit you own.
Which Navien do you have?
On the 2 series that report code E407, the model number on the rating label starts NPE — match yours against the 13 listed below. You can also use the full model-number decoder.
Where this comes from
Source: Navien NPE-2 User's Information Manual (p. 17), read Aug 2026 — the manufacturer's own documentation for this Navien range.
Source: Navien NPE Operation Manual (NPE-A / NPE-S) (p. 16), read Aug 2026 — the manufacturer's own documentation for this Navien range.
Parts, guides and symptoms behind code E407
Code E407 on these 2 Navien series points at 1 guide, 1 part and 1 symptom, each named by the manufacturer's own procedure for this code.