42
Packaged and Split Rooftop Ventilator
© 2021 Mitsubishi Electric US, Inc.
Premi
Sys
NOTE: Unit is equipped with a phase loss/phase reversal control. If system does not start, check phase of electrical
supply.
The first step in troubleshooting a refrigeration circuit is to examine the microprocessor controller and digital scroll
compressor controller (if present) and see if there is a fault code. The next step is to check airflow conditions
(e.g. improper ductwork, atypical wet bulb / dry bulb, etc.). After these steps have been eliminated, proceed with
troubleshooting by following this guide.
Troubleshooting – Packaged DX Refrigeration Circuit
TROUBLESHOOTING NOTE
Before any components are changed on the
refrigeration system, the cause of the failure must be
identified. Further problems will exist unless the true
cause or problem is identified and corrected.
IMPORTANT
Do not release refrigerant to the atmosphere! If
required service procedures include the adding or
removing of refrigerant, the service technician must
comply with all federal, state and local laws. The
procedures discussed in this manual should only be
performed by a qualified EPA Certified Technician.
Symptom
Possible Cause
Corrective Action
Compressor
will not run
or does not
start
Open disconnect switch or circuit
breaker.
Close switch and/or breaker.
Compressor contactor not closing.
Check voltage to contactor coil, transformer, slave relay,
system. Replace parts as necessary.
Blown fuse or tripped breaker.
Check for reason and repair. Replace fuse after correcting
problem.
Low line voltage.
Check line voltage. If more than 10% from compressor
marking, correcting is necessary.
Compressor motor protector open.
Motor thermal protector automatically resets. Allow time (2
hours) for compressor to cool down so protector will reset.
Restart and check for reason overheat occurred.
Compressor defective.
Check motor for open circuit, short circuit, grounded
windings, or burn out. Compressor may be seized; check
refrigerant. If necessary, replace compressor.
High pressure switch open
or defective.
If manual reset (high pressure), reset switch. (Switch
opens at 610 psi and will not reset above 420 psi for
R-410A). Replace if defective.
Low pressure switch open
or defective.
See switch label for setpoint.
Open room thermostat or control.
(No cooling required).
Check room temperature. If temperature is proper, wait for
thermostat to close.
Loose wiring.
Check all wire terminals and tighten as necessary.
Compressor
starts but
cuts out on
low pressure
Low pressure
switch cuts
out at
50 PSIG
(PDX) or 25
PSIG (ASHP)
Low or loss of refrigerant charge.
Check refrigerant pressures and temperatures
(subcooling.)
Airflow restricted.
Check for dirty evaporator coil, dirty filters, dampers
closed, iced evaporator coil, improper belt, check motor
amps, check duct design.
Restriction in refrigerant line.
Check refrigerant pressures, look for frosted tubing and
components indicating a restriction. Check pressure drop
across the filter drier.
Defective low pressure switch.
Replace.
Always have a completed Pre-Start-Up Checklist and Start-Up Checklist prior to requesting parts or service information.
