• High-pressure SW activated although the detected pressure was lower (3.03 MPa or below) than the high-pressure SW activation pressure: Undershift
• High-pressure SW failed to activate although the detected pressure was higher (3.43 MPa or above) than the high-pressure SW activation pressure: Overshift
• The saturation temperature at the detected pressure is 5°C or more below the highest indoor-unit E1 temperature continuously for 30 minutes.
• High-pressure sensor disconnected or open circuit.

1. High-pressure sensor malfunction
2. Failure to connect the connector to the outdoor unit PCB
3. Failure to open the service valve
4. Clogged tubing
5. Valve leakage
6. Over-charging
7. Outdoor unit PCB failure
8. Electrical noise

1. High-pressure sensor failure

• Check the sensor resistance value. (Use a tester and measure the resistance between sensor No. 1 and No. 3)
  Resistance of less than 10kΩ indicates a short circuit or other trouble.
  Resistance of 10 kΩ - 200 kΩ is normal. Resistance of more than 200 kΩ indicates an open circuit or other trouble.
• Connect a gauge to the high-pressure outlet and check for changes in the value dispalyed by the monitoring software, and for large deviation of the gauge pressure.
• During heating, check whether the temperature is lower than the highest indoor-unit E1 temperature.
  * The pressure detected by the high-pressure sensor is the highest pressure in the system. Therefore during heating the converted saturation temperature will never be lower than any indoor-unit E1 temperature. During cooling this temperature will never be lower than the outdoor unit liquid temperature.

2. Failure to open the service valve, clogged tubing, valve leakage, over-charging.
In all of these cases an alarm occurs when there are rapid pressure fluctuations and tracking of the detected pressure is poor.

• Check the open/closed status of the valve.
• Check for clogging of the tubing.
  To check for clogging, disconnect the high-pressure sensor from the PCB and check whether the high-pressure SW activates.
• Check for valve leakage and over-charging.
  When valve leakage or over-charging occurs, refrigerant is likely to accumulate in the oudoor units or indoor units, resulting in a sudden rise in pressure at start that occurs before the refrigerant in the heat exchanger is discharged.
  * The representative valves to check are the liquid valves and mechanical valves.

3. Outdoor unit PCB failure

• The check items are the same as for a high-pressure sensor malfunction.
  A normal PCB is needed to determine whether the problem is a PCB failure or a pressure sensor malfunction. If an abnormality was found at the check items for a high-pressure sensor malfunction, first try replacing the PCB and check again.
  
  Trouble is corrected: Outdoor unit PCB failure
  Trouble is not corrected: High-pressure sensor malfunction

1. Replace the high-pressure sensor.
Caution:
Because the high-pressure sensor connection employs a Schrader-type valve, it can be removed and replaced. However, the high-pressure sensor can be easily damaged by high voltage; therefore use sufficient caution with regard to static electricity.

2. Replace the PCB.

3. Correct the locations of problems in the refrigeration cycle.

• Correct locations where clogging or leakage has occurred.
• In the case of over-charging, recover refrigerant. (Adjust the amount of refrigerant).
  ∗ Guide for over-charging
  Be sure to connect the gauge to the high-pressure pressure outlet when checking for over-charging.
  During cooling:
  The following does not apply when outdoor air temperature is low or when fan speed is controlled. When both compressor 1 and compressor 2 are operating, and the fan mode is 14 (maximum fan speed), then the high pressure saturation temperature should be approximately 15°C above the outdoor air temperature. If it is 5°C or more above this level, then it is possible that over-charging may have occurred.
  During heating:
  There is an indoor unit where refrigerant flow is poor (E1 temperature and discharge temperature are low), and the mechanical valve of that unit is opened to 300 pulses or more, and the E1 temperature is close to room temperature. However be aware that this kind of data results often when there is a height difference between indoor units. Reducing the amount of refrigerant will improve the refrigerant flow, however reducing it too much will increase the likelihood of alarms related to low oil level (scroll-side), the low pressure SW, and discharge temperature. Use caution.