Navien NPE-A / NPE-S Code E421
Cold water inlet thermistor 1: open or short circuit. The first cold water inlet sensor is open circuit or shorted. The unit uses inlet temperature to work out how hard it has to fire, so a dead inlet sensor shows up as water that is not the temperature you asked for.
Part replacementThe verdict
Replace a part
Inexpensive sensor, ordinary service call. Most of the cost is the visit rather than the part.
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
- Book a service call. There is no owner-side check for this code in Navien's manual.
Code E421 on other Navien series
Other Navien series report code E421 under the same fault name, but each manual is transcribed separately and they do not read alike:
Don't confuse this with
3 other Navien NPE-A / NPE-S codes point at a thermistor 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: