Navien water heater shows E004 and will not run; often associated with power issues or control failure.

Safety: Turn OFF power at the breaker before accessing internal components. Do not touch exposed terminals with power applied.

Steps:

• 1. Check household power: Verify the outlet or circuit feeding the Navien has stable 120V AC (in North America) using a plug-in tester or multimeter (if you are qualified). Ensure the breaker is not tripping.
• 2. Inspect power cord and connections: With power OFF, remove the front cover and check that the power cord is securely connected to the terminal block and that there are no burnt or discolored wires.
• 3. Reseat PCB connectors: Gently unplug and replug the low-voltage harnesses on the main PCB one at a time to ensure good contact. Do not force connectors.
• 4. Surge protection: If the unit is not on a surge protector, consider adding one after repair to prevent future PCB damage.

If E004 persists after verifying stable power and secure connections, the main control PCB is likely defective and should be replaced by a Navien-authorized technician, as configuration and combustion tuning may be required after replacement.