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.
If you have significant water pressure, the microswitch may not be fully closed when your water heater is at rest.