Pioneer YN018GMFI16M2D Service Manual page 78

9.4.2.12 P4(Temperature protection of compressor discharge) error diagnosis and solution.
Error Code
Malfunction decision
conditions
Supposed causes
Trouble shooting:
P4
When the compressor discharge temperature(Tp) is more
than 115℃ for 10 seconds, the compressor will stop
restart till Tp is less than 90℃.
● Refrigerant leakage
● Wiring mistake
● The discharge temperature sensor faulty
● Outdoor PCB faulty
and