Refrigeration Compressor Problems: Causes, Checks and Next Steps

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When a walk-in, reach-in or freezer stops holding temperature, the compressor usually gets the blame. Often it is the victim rather than the cause: a dirty condenser, a failed fan, a weak electrical part or a refrigerant leak pushes it until it overheats, trips or fails. This page explains the common compressor problems in commercial refrigeration, what a manager can safely check, what a technician looks for, and how to think about repairing a compressor versus replacing more of the system. It is part of our Commercial Refrigeration Help Center.

How the compressor fits into the system

The compressor pumps refrigerant vapor from the evaporator (the coil and fans inside the box) to the condenser (the coil and fan that throw heat out, on the unit itself, on a roof or in a machine room). A metering device such as a TXV feeds liquid refrigerant back into the evaporator. Around the compressor sit its controls: the thermostat or controller that calls for cooling, a contactor or relay that switches power, start components on single-phase compressors (capacitors and a start relay), an internal or external overload, high and low pressure switches, and on many systems a defrost timer or controller. A problem anywhere in that loop shows up as a compressor symptom.

Symptom by symptom

It will not start, or it hums and clicks off

First confirm the basics: the breaker, the disconnect switch at the condensing unit, and whether the thermostat or controller is actually calling for cooling (a unit in defrost will not run the compressor). If the compressor hums for a few seconds and then clicks off, common causes are a failed start or run capacitor, a bad start relay, low voltage, a pitted contactor, or an overload that has tripped because the compressor is hot. A compressor that is mechanically stuck will do the same thing. These all need a technician with a meter. Repeated attempts to start a compressor that cannot start overheat its windings.

It runs all the time, but the box is warm

A compressor that never shuts off is working but not moving enough heat. Turbo Air's walk-in system troubleshooting table lists "Dirty condenser coil" as a cause of high discharge pressure, and "Lack of refrigerant", "Evaporator dirty or iced" and a "Clogged liquid line filter drier" as causes of low suction pressure (Turbo Air). In a kitchen that translates to a few usual suspects: a condenser clogged with dust and grease, a condenser fan that has stopped, an evaporator coil packed with ice, a refrigerant leak, or a door that is not closing. A technician works through those in order before condemning the compressor.

Short cycling

Short cycling means the compressor starts and stops far more often than normal. Heatcraft's condensing unit manual lists short cycling on the low pressure switch in cold weather and gives three causes: the "Low pressure switch not correctly adjusted", "Lack of refrigerant", and "Low ambient kit not installed" (Heatcraft). On Bay Area rooftops that can show up on a cold winter night. Other common causes technicians find are an overload tripping because the compressor is overheating, a controller or thermostat with too narrow a differential, a sensor in the wrong spot, and an electrical fault that drops the contactor. Each start stresses the motor, so short cycling shortens compressor life even when the box is still cold.

It trips on overload or high pressure on hot days

Heat is the enemy of compressors. When the condenser cannot shed heat, head pressure and motor temperature climb until a safety opens. Manitowoc describes the same effect on its ice machines: a dirty condenser "restricts airflow, resulting in excessively high operating temperatures," which "shortens component life" (Manitowoc). The fix is usually outside the compressor: cleaning the condenser, repairing the fan, clearing whatever is blocking airflow, or dealing with a unit that sits in a space that gets too hot.

It is loud

Knocking at startup, rattling, or a loud buzz from a compressor each point to different faults. See our refrigeration noises and alarms page for a sound-by-sound table.

Hot kitchens and condenser airflow

Self-contained units such as prep tables, undercounters and chef bases often sit right beside fryers, ovens and grills, pulling in hot, greasy air. Their condensers load up faster than anything else in the building. Remote condensing units on roofs collect dust, leaves and grease from kitchen exhaust. A few rules keep them breathing:

• Keep boxes, towels and supplies away from grilles and vents, and never block the discharge air.
• True's service manual suggests a simple check: "use a flashlight to verify that you can see through the coil and observe the condenser fan blade spinning" (True). If you cannot see light through the coil, it is overdue.
• Manufacturers call for condenser cleaning anywhere from "at least every three months (90 days)" on Turbo Air reach-ins (Turbo Air) to semi-annually on Heatcraft condensing units, with quarterly checks in between (Heatcraft). Greasy kitchens need the short end. Heatcraft also warns that foam coil cleaner "should not be an acid based cleaner."
• True notes that routine care and maintenance are not covered by its warranty (True), so a failure caused by a clogged condenser can come out of your pocket.

Safe checks a manager can make

1. Read the thermometer in the unit and write down the temperature and time.
2. Check the breaker and the disconnect near the condensing unit. Reset a tripped breaker once. If it trips again, leave it off and call.
3. Check the controller or thermostat setting, and whether the unit is in defrost.
4. Look at the condenser with a flashlight: is the coil visibly dirty, and is the fan turning when the compressor runs?
5. Clear anything blocking airflow around the unit or rooftop condenser.
6. Look inside the box: is the evaporator coil iced, and are its fans running? Is the door closing and sealing?
7. Listen and note: does the compressor hum and click off, run nonstop, or start and stop every few minutes?

Stop there. Opening electrical panels, adding refrigerant or touching the sealed system is technician work, and anyone who opens a refrigerant circuit must hold EPA Section 608 certification.

What a technician checks

We measure suction and discharge pressures, superheat and subcooling to see whether the system is short of refrigerant, overcharged, restricted or starved of airflow. We check voltage at the compressor while it starts, amp draw against the data plate, the contactor, capacitors and start relay, and the compressor windings. If a compressor has burned out, we check the oil and refrigerant for acid, because a new compressor installed into a contaminated system will fail again; burnout cleanup usually means new filter driers and careful evacuation. Most important, we look for why it failed: a leak, a failed fan, a flooding TXV, an iced evaporator or chronic overheating. Our older post on what causes compressors to fail covers the general mechanisms, and how technicians find refrigerant leaks explains leak detection.

Repair the compressor or replace more?

A compressor can often be replaced on its own, and a failed capacitor, contactor or fan motor is a small repair that saves the compressor. The bigger decision comes when the compressor itself is gone. Things to weigh:

• Age and condition of the rest of the system: a new compressor on a corroded condenser coil with a leaking evaporator is a short-lived fix.
• Refrigerant: R-22 production and import ended in 2020, though reclaimed R-22 is still available (EPA). In California, newly produced bulk HFCs with a GWP above 2,200 cannot be sold since January 1, 2025, while certified reclaimed refrigerant is exempt (HSC 39735). That affects older R-404A and R-507A systems, which can be serviced but only with reclaimed refrigerant.
• Leak rules: federal HFC leak repair rules apply to commercial refrigeration with 15 lb or more of refrigerant from January 1, 2026, with a 20 percent leak-rate trigger and a 30-day repair window (40 CFR 84.106). California's Refrigerant Management Program requires leaks in systems over 50 lb of high-GWP refrigerant to be fixed within 14 days of detection (CARB).
• What replacement means: a new condensing unit has to meet the refrigerant rules in force when it is installed. Our refrigerant rules guide explains them, and our condensing unit replacement service covers the work.

Existing equipment is allowed to keep running; EPA says there are no requirements to stop using equipment currently in use (EPA). For costs, see our walk-in cooler repair cost guide rather than a number on this page, and our repair or replace guide for the full decision. Every repair we do carries a 1-year parts and labor warranty.

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Written and reviewed by Jim McKinny, owner of Cool Aid Air Conditioning & Refrigeration — California C-20 and C-38 licensed, EPA Universal certified, second generation of a family business serving the Bay Area since 1966. Every job we complete is backed by our 1-year parts & labor warranty.