The E6 error appears when the room or evaporator temperature sensor are out of power or short-circuited. It can be fixed in the following way:
Make sure that the room temperature sensor connections are ok, and resistance is correct. If there is no problem with the sensor, the control board needs to be repaired or replaced.
Not unlikely, the unit is using the NTC thermistor as a thermometer sensor. At room temperature, the sensors should read 1kOhm and 10 kOhm respectively. Resistance will decrease as the temperature increases.