Navien NPE-A / NPE-S Code E785
Flow switch or flow sensor fault. A fault at the flow switch or flow sensor. On the NPE-2 Navien narrows this code to installations with an air handler; on this generation it is listed without that qualifier, so it applies more broadly.
Part replacementThe verdict
Replace a part
A flow-side component. Clean the inlet filter first — it will not clear the code, but debris in the water path is part of how these fail.
Typical cost: $150–$600
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
- Clean the cold water inlet filter.
- Book a service call.
Code E785 on other Navien series
Other Navien series report code E785 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: