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
Other Navien series report code E740 under the same fault name, but each manual is transcribed separately and they do not read alike:
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: