Navien NPE-A / NPE-S Code E740
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.
Part replacementThe verdict
Replace a part
An external sensor and a service call. Low urgency unless the system depends on it for outdoor reset control.
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
- If the sensor is mounted outside and reachable, check it has not been damaged, painted over, or wrapped in insulation.
- Book a service call.
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-A / NPE-S 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: