Panasonic WH-UD28AE Service Manual page 58

16.5.16. Indoor High Pressure Protection (H98)
Error code will not display (no TIMER LED blinking) but store in EEPROM
Malfunction Decision Conditions:
During operation of heating, pressure 4.0 MPa and above is detected by outdoor high pressure sensor.
Malfunction Caused:
1. Dust accumulation on the indoor unit heat exchanger.
2. Detection error due to faulty indoor pipe temperature sensor.
3. Detection error due to faulty indoor unit PCB.
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