Symptom Possible Cause
Corrective Action
Low
discharge
pressure
Insufficient refrigerant charge.
Check subcooling, check for leak. Repair leak and add
refrigerant.
Defective or improperly adjusted
expansion valve.
Check superheating and adjust thermal expansion
valve.
Low suction pressure.
See “low suction pressure”.
Faulty condenser temperature controls.
(Condensing fan cycle switch).
Check condenser controls and reset to obtain desired
condensing temperature.
Compressor
short cycles
Thermostat location or controls
malfunction.
Check thermostat, check heat anticipator setting.
Improper refrigerant charge.
Check subcooling, verify superheat.
Defective high or low pressure control.
Check high or low pressure switch.
Liquid floodback.
Possible tight bearings, see page 30.
Defective expansion valve.
Check thermal expansion valve and superheat.
Poor air distribution.
Check ductwork for recirculating.
High discharge pressure.
See “high discharge pressure”.
Leaking discharge valves in
compressor.
See “high suction pressure”.
Low airflow at evaporator(s).
Check blower operation and airstream restrictions.
Incorrect unit selection (oversized).
Contact factory.
Compressor
loses oil
Refrigerant leak.
Check system for leaks. Repair leaks and add
refrigerant.
Short cycling.
Check low pressure control settings.
Refrigerant flood back.
Check thermal expansion valve setting. Check for
refrigerant overcharge. Check crankcase heaters.
Improper piping or traps.
Verify proper piping slopes.
Reheat flush cycle inadequate.
Contact factory.
Not enough
cooling
or lack of
cooling
Refrigeration undercharged.
Check subcooling. Adjust charge, if necessary.
Dirty filter or evaporator coil.
Check filter, coil and airflow.
Dirty or clogged condenser coil.
Check coil and airflow.
Air or other non-condensables in
system.
Check equalized high side pressure with equivalent
outdoor temperature.
Defective compressor.
See “high suction pressure”.
Restriction in suction and liquid line.
Check for restrictions in refrigerant circuit.
Control contacts stuck.
Check wiring.
Excessive load.
Add more equipment or reduce room load.
Liquid line
is frosted
or wet
Restriction in liquid line.
Clear restriction upstream of point of frosting.
Troubleshooting – Refrigeration Circuit
Always have a completed Pre Start-Up Checklist, Unit Start-Up Checklist and Optional Accessories Checklist prior to
requesting parts or service information.
34
Model MPX Make-Up Air Unit
Model XMPX Make-Up Air Unit
