The E0 error code pertains to the outgoing water temperature sensor.
This can typically be corrected by verifying the connection of this sensor with the computer board. The sensor is located on the outgoing water pipe. To access the sensor, unplug the water heater from the electrical outlet and remove the cover by unscrewing the four screws that hold it in place. After doing so, find the sensor's connection to the computer board, unplug the connector, and reconnect it. The connecter is the fourth connection from the top on the computer board. You can now plug the water heater back and attempt to run it as usual.
If you would like to see images to help you find this connection, please download the attached comprehensive troubleshooting guide.
If this does not fix your issue, please contact technical support at 866-356-1992 as it may be necessary to replace your sensor.