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
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: