Navien tankless water heater code E740
Outdoor temperature sensor fault. Code E740 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 E740 means on each Navien series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| NPE-2 | Outdoor temperature sensor fault | Part replacement | $150–$450 |
| NPE-A / NPE-S | Outdoor temperature sensor fault | 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 E740 — 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 E740 agree and where they part company.
NPE-2 — Outdoor temperature sensor fault
The outdoor temperature sensor is not reporting correctly. Navien flags the code as applying where an AHU is installed — that sensor is used to adjust output against outdoor conditions, so it belongs to installations with air handling attached rather than to a plain domestic hot water setup.
NPE-A / NPE-S — Outdoor temperature sensor fault
The outdoor temperature sensor is not reporting correctly. On the NPE-2 Navien qualifies this code as applying where an air handler is installed; on this generation it is listed plainly, without that qualifier.
Does it matter which Navien you have?
NPE-2 and NPE-A / NPE-S all route code E740 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 E740, 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. 18), 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 E740
Code E740 on these 2 Navien series points at 1 part, each named by the manufacturer's own procedure for this code.