The E0 error code pertains to the outgoing and incoming water temperature sensor.
This can typically be corrected by verifying the connection of these sensor(s) with the computer board. The outlet temperature sensor is located on the outgoing water pipe, which is located inside of your EL22 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 trace the outlet temp sensor's connection to the computer board, unplug the connector, and reconnect it. The connector goes to the top right corner of the computer board.
The inlet water temperature sensor is located on the top of the water flow meter. To remove this sensor, you must first remove the water flow meter assembly.
First, disconnect the wiring from the flow meter.
Next, remove the 2(two) screws on the bottom of the unit holding the water inlet pipe to the unit.
After that is done, remove the 2(two) silver clips holding the flow meter in place.
While holding the water inlet pipe, remove the bottom clip and set the inlet pipe aside. Remove the upper clip on the water flow meter and slowly lift the flow meter off of the copper pipe the meter is attached to.
Now, there will be a silver bracket holding a white wire to the flow meter. Remove the single screw and slide the bracket off the sensor. Slowly remove the sensor from the assembly and take care to remove the double o-ring inside if it does not come out with the sensor.
Follow the wiring for the sensor and disconnect from the wiring located on the upper right area of the computer board.
Install the new sensor into the assembly and install the bracket with the single screw.
Reconnect the sensor to the board and reverse the rest of the process for re-assembly.
Take care when reinstalling the flow meter to the copper tubing not to tear the o-ring on the copper pipe. The o-ring is opaque in color (clear/white) When reinstalling the silver clips, the larger diameter will clip to the meter and the smaller diameter of the clip will go to the copper tubing. Once the installation is complete, you can now plug the water heater back in and attempt to run it as usual.