E421
Navien
External Control / Thermostat Communication Fault
Display shows E421; unit may not respond to external controls properly.
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Safety Warning
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.
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Possible Causes
Incorrect thermostat wiring, Shorted external control terminals, Incompatible control device, Faulty PCB input circuit
How to Fix / Troubleshooting
Safety: Turn OFF power before working on control wiring.
Steps:
- 1. Verify wiring: Check that external thermostat or control wires are connected to the correct Navien terminals and are not shorted together.
- 2. Remove external control: Temporarily disconnect external control wires and reset the unit. If E421 clears, the issue is with the external device or wiring.
- 3. Check compatibility: Ensure the thermostat or control being used is compatible with Navien's control input specifications.
If E421 remains with no external wiring connected, the PCB input circuit may be defective and require replacement.
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Repair Difficulty
Medium
3/5
Required Part
Main Control PCB
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