The E0 error code can be the result of a power surge or power outage or interruption.
This code can frequently be resolved by turning off the unit and unplugging it from the wall for 30 seconds. This needs to be done with the faucets turned off and no water flowing through the unit. If water is flowing through the unit, the computer will not clear the code.
Lastly, when you plug the water heater, then the E0 code immediately appears, please download the guide for the E0 error code. This can result from the water heater thinking it has water running through it when it is initially powered on.
If you have significant water pressure, the microswitch may not be fully closed when your water heater is at rest. The comprehensive trouble shooting guide has pictures of the microswitch with examples of what it looks like open and what it looks like when it is properly closed.