Navien unit displays E407; often associated with DHW outlet sensor or mixing issues.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Faulty DHW outlet thermistor, Loose sensor wiring, Internal mixing valve malfunction, Scale buildup around sensor location

Safety: Turn OFF power and allow hot water lines to cool before touching.

Steps:

1. Locate DHW outlet sensor: Identify the thermistor on the domestic hot water outlet line and ensure it is firmly attached.
2. Check wiring: Verify the sensor connector is secure on the PCB and the cable is intact.
3. Descale if needed: If there is heavy scale on the outlet piping, consider descaling the unit to improve sensor accuracy.

If E407 persists, the sensor or internal mixing valve may need replacement by a technician.

Medium 3/5

DHW Outlet Temperature Sensor

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