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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).