The E03 error code on your ZERO BREEZE Mark 2 air conditioner indicates low voltage protection. This means that the unit has shut off, the buzzer alarm is sounding, and the tail LED light is flashing due to a depleted battery. To fix this issue, you need to charge the battery to meet the required input voltage for the Mark 2 AC. In normal mode, the output voltage should be above 18.6V, and in car mode, it should be above 22.3V.