DIAGNOSIS: That's a temperature sensor fault. The unit's internal sensor that monitors evaporator coil temperature has failed or is disconnected.
FIX: Unplug the unit for a full 5 minutes to force a hard reset. This clears the error memory. Plug it back in. If E1 returns immediately, the sensor is bad and requires professional repair. There is no user-serviceable part for this.
IF THAT DIDN'T WORK: If the reset seems to work but E1 comes back after a few hours of operation, you likely have a failing control board sending incorrect signals. This also means a service call.
HEADS UP: Don't ignore this error. Running with a faulty sensor can cause the coil to freeze up, leading to water damage from overflow.