Panasonic WH-UD28AE Service Manual page 56

16.5.14. Compressor Low Pressure Protection (H66)
Error code will not display (no OFF/ON Control Panel LED blinking) but store in EEPROM
Malfunction Decision Conditions:
During operation of heating, pressure 0.2 MPa and below is detected by the outdoor low pressure sensor.
Malfunction Caused:
1. Dust accumulation on the outdoor unit heat exchanger.
2. Air short circuit.
3. Refrigerant shortage (refrigerant leakage).
4. Detection error due to faulty outdoor unit main PCB.
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