Boiler shows E407 and will not operate, often after connecting external controls

Safety first: Turn off power to all connected units before changing communication wiring.

• Step 1 – Verify wiring: Check the communication cable between the boiler and external controller or other boilers. Ensure polarity and terminal assignments match Navien diagrams.
• Step 2 – Check addresses: Confirm each boiler in a cascade has a unique address and that the master/slave roles are correctly set.
• Step 3 – Reseat connectors: Unplug and firmly replug communication connectors at both ends.
• Step 4 – Isolate the fault: Disconnect the external controller and see if the boiler operates normally. If so, the controller or its wiring is likely at fault.

Persistent E407 errors may require Navien technical support or controller replacement.