Error List of Air Conditioning   System UO-Scarcity of Refrigerant Lack of refrigerant and refrigerant clogging (incorrect piping) Faulty thermistor Faulty Low Pressure Sensor Faulty outdoor unit main PCB U1-reverse phase,Open phase Power supply reverse phase t phase open phase Defective Outdoor Unit PCB (A1P) U2-Malfunction Power supply or instantaneous power failure Abnormal power supply