Navien tankless water heater code E421
Cold water inlet thermistor 1: open or short circuit. Code E421 means the same thing on all 2 Navien series documented here — but the fix, and what it costs, is still worth reading for your exact unit.
What code E421 means on each Navien series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| NPE-2 | Cold water inlet thermistor 1: open or short circuit | Part replacement | $150–$450 |
| NPE-A / NPE-S | Cold water inlet thermistor 1: open or short circuit | Part replacement | $150–$450 |
Each row opens the full page for that generation. We hold manuals for NPE-2 and NPE-A / NPE-S, and each one gets its own reading of code E421 — what Navien documents, what you can safely check yourself, and what the repair should cost.
What each manual actually says
The table above compares 2 series in four columns, which is enough to route you and not enough to explain anything. This is the Navien wording behind each row, one entry per manual, so you can see where the 2 readings of code E421 agree and where they part company.
NPE-2 — 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.
NPE-A / NPE-S — 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.
Does it matter which Navien you have?
NPE-2 and NPE-A / NPE-S all route code E421 the same way: a replacement part, at $150–$450. That makes it one of the Navien numbers you can act on before working out which unit you own.
Which Navien do you have?
On the 2 series that report code E421, the model number on the rating label starts NPE — match yours against the 13 listed below. You can also use the full model-number decoder.
Where this comes from
Source: Navien NPE-2 User's Information Manual (p. 17), read Aug 2026 — the manufacturer's own documentation for this Navien range.
Source: Navien NPE Operation Manual (NPE-A / NPE-S) (p. 16), read Aug 2026 — the manufacturer's own documentation for this Navien range.
Parts, guides and symptoms behind code E421
Code E421 on these 2 Navien series points at 1 part and 1 symptom, each named by the manufacturer's own procedure for this code.