YORK YLAA — SYS 1 MP/HPCO FAULT SYS 2 MP/HPCO FAULT

The panel shows SYS 1 MP/HPCO FAULT for system 1 and SYS 2 MP/HPCO FAULT for system 2 — the number after SYS identifies the system. This wording is the only variant of the message. An earlier trip of the same protection is displayed as MP/HP INHIBIT; the MP/HPCO FAULT wording appears only once the system has locked out.

MP/HPCO stands for Motor Protector / Mechanical High Pressure Cutout — the protection that guards the compressor motor from overheating and the system from experiencing dangerously high discharge pressure. The control logs it as System Fault Code 18. The fault condition is present when the CR1 (SYS 1) or CR2 (SYS 2) relay de-energizes because the HP switch or the motor protector has opened, so the microboard reads 0 VDC on its input; the condition is cleared when a 30 VDC signal is restored to the input. Two devices can open that circuit: the internal motor protector and the mechanical HP switch. The compressor discharge temperature sensor can also open the motor protector circuit when it senses high internal scroll temperature — for example with a low refrigerant charge or superheat too high.

• Inadequate cooling of the scrolls — refrigerant charge low, or superheat too high, detected by the compressor discharge temperature sensor.
• Compressor motor overheating.
• Dangerously high discharge pressure.

1. Note how many faults have occurred: the first two faults of this protection are displayed as MP/HP INHIBIT and the system is not locked out — MP/HPCO FAULT appears only after the third fault in 90 min.
2. Check the refrigerant charge; the manual lists a low charge among the causes of inadequate cooling of the scrolls.
3. Check superheat: superheat too high is what the compressor discharge temperature sensor sees as high internal scroll temperature, and it opens the motor protector circuit.
4. Check for dangerously high discharge pressure — the condition the mechanical HP switch protects the system against.

• Internal motor protector: opens at 185°F to 248°F (85°C to 120°C); resets automatically.
• Mechanical HP switch: opens at 585 PSIG ± 10 psig (27.92 barg); closes again at 330 psig ± 25 psig (22.75 barg ± 1.72 barg).
• Fault detection: 0 VDC on the microboard input with CR1 (SYS 1) or CR2 (SYS 2) de-energized; cleared when a 30 VDC signal is restored to the input.
• Lockout logic: MP/HP INHIBIT on the first two faults; MP/HPCO FAULT and lockout after the third fault in 90 min.
• Cool-down: internal compressor contacts open for 30 min after a shutdown before the system is permitted to restart.
• Logged as System Fault Code 18.

On this message the system is locked out and will not restart. Clear a lockout by turning the system switch OFF and then back ON (under OPTIONS key). The fault condition itself is cleared when the 30 VDC signal is restored to the input. Whenever the motor protector or the discharge sensor shuts down a compressor and the system, the internal compressor contacts open for 30 min so the motor or scroll temperatures can dissipate heat and cool down; after 30 min the contacts close and the system is permitted to restart. The internal motor protector resets automatically, and the mechanical HP switch closes again at 330 psig ± 25 psig.