If you see the error code E04 on your Navien water heater, it means that the temperature sensor or thermistor is not working correctly. This can happen for a number of reasons, including a loose connection, a bad sensor, or a problem with the control board.
If you see this error code, you'll need to troubleshoot the problem to figure out what is causing it. First, check the connections to the sensor and make sure they are tight. Next, test the sensor with a multimeter to see if it is working correctly. If the sensor is bad, you'll need to replace it. Finally, check the control board for any signs of damage or corrosion.