Navien E407

What does Navien E407 mean?

E407 indicates the hot water outlet thermistor has reported an open or short circuit condition. The PCB uses this thermistor to monitor delivered water temperature and modulate the burner; when the sensor's resistance is outside the expected range for any water temperature, the control board treats the reading as a fault and shuts the unit down to prevent scalding or damage.

Symptoms

• Unit displays E407 and stops producing hot water.
• Water temperature may be erratic or incorrect just before the error appears.
• Error persists after reset attempts; the thermistor fault is continuous.
• Hot water output cut off completely at the outlet.

Common causes

• Thermistor probe has failed open or short internally.
• Wiring harness connector at the thermistor is corroded, loose, or broken.
• Water leak near the thermistor has caused connector corrosion.
• PCB thermistor input circuit has failed.

Diagnostic steps

1. 01

   Power down the unit

   Switch off power and shut off the cold water supply before accessing internal components. Allow the unit to cool if it has been running.

2. 02

   Locate the hot water outlet thermistor

   Consult the wiring diagram in the service manual for your model (NPN-199E section) to identify the hot water outlet thermistor mounting location on the outlet pipe. It typically connects via a two-wire push-on connector.

3. 03

   Inspect the wiring harness and connector

   Disconnect the thermistor connector and inspect both the thermistor-side and harness-side pins for corrosion, bent contacts, or broken wire leads. Reseat the connector firmly and check for proper latching.

4. 04

   Test thermistor resistance

   With the connector disconnected, measure resistance across the thermistor terminals with a multimeter. Cross-reference the reading against the resistance-temperature table in the service manual. A reading of 0 Ohms (short) or infinite resistance (open) confirms thermistor failure.

5. 05

   Replace the thermistor if defective

   Order the OEM replacement thermistor for your Navien model. Install with the same immersion depth and orientation, reconnect the harness, restore power, and run a test cycle to confirm E407 is cleared.

When to call a professional

If replacing the thermistor does not clear E407, the fault may lie in the PCB's thermistor input circuit. PCB replacement on Navien units requires reconfiguring system settings after installation and verifying correct temperature modulation — contact Navien Technical Support at 1-800-519-8794 or dispatch a licensed technician.