The E0 error code pertains to the outgoing water temperature sensor.
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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, which is located inside of your 45 model-on the bottom, left corner behind the computer board. To access the sensor, unplug the water heater from the electrical outlet, turn your gas source off to the heater, and remove the cover by unscrewing the four screws that hold it in place. After doing so, then remove your computer board by removing the two screws located at the bottom corners of the gray plastic saddle that houses the computer board. Once you have located the outlet temp sensor (which is a white cord, with a brass nut that connects to the bottom of the copper pipe), then find trace the outlet temp sensor's connection to the computer board, unplug the connector, and reconnect it. The connecter goes to the top right corner of the computer board. Once this is complete, you can now plug the water heater back and attempt to run it as usual.