42
McQuay OM 751
Diagnostics and Service
Space Temp Sensor Failure (
F0
)
The Space Temp Sensor Failure fault occurs when the UVC detects open or short conditions
from the sensor.
Effect:
•
Space fan de-energizes (unless in emergency heat mode).
•
Compressor immediately de-energizes.
•
Outdoor fan (if present) de-energizes.
•
Outside air damper is forced closed.
•
Electric heat stages are de-energized.
•
Fault is indicated.
DX Pressure Fault (
F1
)
The DX Pressure Fault occurs when the UVC detects a switch open condition from the
refrigerant pressure switch.
Effect:
•
Compressor immediately de-energizes.
•
Outdoor fan (if present) de-energizes.
•
Fault is indicated.
Compressor Envelope Fault (
F2
)
The UVC monitors refrigerant temperatures. The Compressor Envelope Fault occurs when the
UVC detects compressor operation that exceeded the allowed operating parameters.
Effect:
•
Compressor immediately de-energizes.
•
Outdoor fan (if present) de-energizes.
•
Fault is indicated.
Cause:
•
Poor air or water flow through the refrigerant coils. Check fans for proper rpm. Check air
filters.
•
If the unit has a three-phase scroll compressor, check for proper electrical phasing.
•
Refrigerant circuit component failure or improper adjustment. Check refrigerant pressures
and TXV adjustment.
•
Coil sensors may have lost proper contact with the refrigerant coil. Check coil sensors.
Discharge Air DX Cooling Low Limit Indication (
F3
)
The Discharge Air DX Cooling Low Limit Indication fault occurs when the UVC detects a
low discharge air temperature (DAT < MCLL) during compressor cooling.
Effect:
•
Compressor immediately de-energizes.
•
Outdoor fan (if present) de-energizes.
•
Fault is indicated (on earlier software versions).
