Navien NPE-2 Code E421
Cold water inlet thermistor 1: open or short circuit. The first of the cold water inlet sensors has an open or shorted circuit. That sensor is how the heater knows how cold the incoming water is, which is what sets how hard it has to fire. Navien's instruction is to contact technical support.
Part replacementThe verdict
Replace a part
A sensor fault: cheap part, service call around it. Until it is fixed the heater is working from an incomplete picture of the incoming water, so expect the outlet temperature to be less steady than usual.
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-2 — not averaged across the brand.
Source: Navien NPE-2 User's Information Manual (p. 17), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note when the unit was last flushed and whether your water is hard — scale on inlet sensors is common.
- Contact your installer or Navien technical support.
Frequently asked questions
Why are there two cold water inlet sensors?
Navien numbers them 1 and 2 and gives them separate codes — E421 and E432. Two readings on the inlet side let the unit cross-check and, on recirculating installations, tell incoming mains water from returning loop water.
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-2 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: