Mitsubishi Heavy air conditioner error U3

• Checking an air conditioner temperature sensor (thermistor)
  How to check the room, coil or discharge sensor when the unit reports a sensor error: measure its resistance and compare with the exact model's table.
• Pressure, low refrigerant and high discharge temperature errors: where to start
  Pressure and discharge-temperature protection codes: tell a low charge from a blockage or poor heat rejection before charging.

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