Navien E421 Error Code

What does Navien E421 mean?

E421 means the cold water inlet 1 thermistor has reported an open or short circuit. This sensor measures incoming cold water temperature so the PCB can calculate the required burner output; without a valid reading, the controller cannot modulate firing rate safely and shuts the unit down. Thermistor failure, wiring damage, or connector corrosion are the typical causes.

Symptoms

• Unit shuts down with E421 and does not produce hot water.
• Error appears immediately at startup rather than after a run period.
• Erratic temperature readings may precede a permanent fault display.
• No response from the unit after multiple reset attempts.

Common causes

• Cold water inlet 1 thermistor probe failed open or short internally.
• Wiring harness connector at the thermistor is corroded or loose.
• Wire lead broken due to vibration or chafing against sheet metal.
• PCB thermistor input channel has failed.

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Diagnostic steps

1. Power down and locate the thermistor

   Switch off the unit and cold water supply. Consult the wiring diagram for the NPN-199E or your specific model to locate the cold water inlet 1 thermistor on the cold-side inlet pipe.

2. Inspect connector and wiring

   Disconnect the thermistor harness connector and inspect both sides for corrosion, bent pins, or broken wire leads. Reseat firmly and check for proper latching before proceeding to resistance testing.

3. Measure thermistor resistance

   With the connector disconnected, place multimeter probes across the thermistor terminals. Compare the reading against the resistance-temperature chart in the service manual at the current ambient temperature. Open (infinite) or short (zero Ohms) indicates a failed thermistor.

4. Check diagnostic value in service menu

   Access the Navien diagnostic mode via the front panel and navigate to the [C] parameter to view the thermistor's live reading. A value that is frozen, stuck at maximum, or reading 0 confirms sensor failure.

5. Replace the thermistor

   Order the OEM cold water inlet thermistor for your model. Install at the same position with identical immersion depth, reconnect the harness, restore power, and verify E421 is cleared through a full heat call.

When to call a professional

If E421 persists after thermistor replacement, the PCB input circuit may have failed. PCB replacement requires downloading configuration settings and recalibrating system parameters after installation — tasks that require a Navien-authorized technician and access to the manufacturer's service software.