E320 signals an OLP sensor issue. The sensor is located in the outdoor unit. First, you can try to reset it by switching it off for 30 seconds or so. If the error appears again, the temperature sensor is down and needs to be replaced. There are online parts stores, where you can order and buy almost any part you need. However, the replacement process can be a challenge because the sensor and the PCB are located deep inside the system, without disassembling which getting to them can be a problem.