Navien NPE-2 Code E740
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.
Part replacementThe verdict
Replace a part
An inexpensive sensor, usually mounted outside the building, and the common causes are as physical as they sound: a damaged cable, water in a connector, or a sensor knocked off its mounting. Hot water generally carries on working.
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. 18), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- If you know where the outdoor sensor is, check it is still mounted and its cable is intact.
- Contact your installer or Navien technical support.
Frequently asked questions
My unit has no outdoor sensor. Why this code?
Either one was fitted and has since been removed or damaged, or the unit is configured as though one exists. Both are worth saying when you call, because they send the technician to different places.
Code E740 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
2 other Navien NPE-2 codes point at a sensor 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: