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
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
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: