YORK YLAA — MP / HPCO FAULT

The panel shows MP / HPCO FAULT. This is the only form of the message the YLAA manuals print — there is no second spelling or variant to watch for.

The message reports that a protective device in the compressor circuit has opened or failed. The manual's fault table lists five conditions behind it: the compressor internal motor protector (MP) is open, an external overload has tripped, the HPCO switch is open, the HPCO switch is defective, or the CR relay is defective. The message itself does not tell you which of the five it is — the checks below separate them.

• The compressor internal motor protector (MP) is open.
• An external overload has tripped.
• The HPCO switch is open.
• The HPCO switch is defective.
• The CR relay is defective.

1. If the compressor internal motor protector (MP) is the open device, verify that the refrigerant charge is not low.
2. For the same MP condition, verify the superheat setting is 10°F to 15°F (5.6°C to 8.3°C).
3. Also for the MP condition, verify that the compressor rotation is correct.
4. Complete the MP checks by verifying that the compressor is not overloaded.
5. If an external overload has tripped, determine the cause and reset it.
6. If the HPCO switch is open, see the separate “High Press. Disch.” fault entry.
7. Replace a defective HPCO switch.
8. Replace a defective CR relay.

• Superheat setting: 10°F to 15°F (5.6°C to 8.3°C) — verify this setting as part of the checks for an open compressor internal motor protector (MP).

The manual gives one reset instruction for this fault, and it applies to a tripped external overload: Determine cause and reset. The other recoveries it names are component replacement — a new HPCO switch or a new relay — and, for an open HPCO switch, the separate “High Press. Disch.” fault entry.