If your commercial refrigerator stopped cooling overnight, the compressor is usually the first thing you suspect. More often than not, it’s not the compressor itself. A dead capacitor, a tripped overload relay, or a wiring issue can produce the exact same symptom: the unit isn’t running, nothing’s cooling, and you’re staring at a warm box full of product.
Here’s how to read what the unit is telling you.
What You’ll Actually Hear (and Not Hear)
A compressor that won’t start gives you distinct clues depending on why it stopped.
Clicking, then silence. The unit tries to start, clicks once or a few times, then goes quiet. That’s the overload protector cutting power to protect the compressor from overheating or overcurrent. It could mean the compressor is struggling to start (worn motor windings, bad start capacitor) or it could mean the compressor got hot for another reason, like low refrigerant or a dirty condenser.
Humming without starting. You hear a steady hum but the compressor never kicks over. Classic sign of a failed start capacitor. The run winding is energized but the start winding isn’t getting the phase shift it needs to spin the motor.
Completely silent. No sound at all when the thermostat calls for cooling. This points to an electrical issue upstream: failed contactor, blown fuse, tripped breaker, or a bad thermostat or control board. The compressor may be fine.
Runs briefly, then shuts off. Starts, runs a few minutes, cuts out. Thermal overload behavior. Usually means the compressor is overheating from a refrigerant problem (low charge, restriction), dirty condenser coils, or high ambient temperatures around the unit.
The Likely Culprits, in Order
Start/run capacitor. The most common failure on older reach-ins and walk-ins. A tech can test one in under a minute with a capacitance meter. If yours is bulging or leaking from the terminals, it’s done.
Overload relay. The overload protector clips onto the compressor body and cuts power if the motor draws too much current or gets too hot. They fail on their own sometimes, but often they’re signaling something else is wrong. Replacing one without understanding why it tripped is a temporary fix.
Contactor or relay. The contactor closes the circuit to send power to the compressor. The contacts can burn or stick. Diagnosing this requires voltage testing at the unit.
Dirty condenser coils. Not a compressor problem by itself, but it causes compressor problems. Clogged coils make the compressor work harder and run hotter. Eventually it trips on thermal overload or burns out.
Low refrigerant. If the system is low (usually from a slow leak), the compressor can overheat or the low-pressure switch will cut it off. You can’t diagnose this without gauges, and purchasing HFC refrigerants requires an EPA 608 certification. Adding refrigerant to a leaking system without fixing the leak first damages the compressor and delays the real repair.
Compressor failure. Motor windings can short or open. The mechanical pump inside can seize. A tech tests winding resistance with a multimeter and reads system pressures with gauges. True compressor failure is the expensive scenario, and also not the first place to look.
What a Tech Will Do
A good technician doesn’t just swap the compressor because nothing’s running. They’ll start by checking incoming voltage, then test the thermostat output, contactor, and capacitor. They’ll put gauges on the system to read suction and discharge pressures, then check winding resistance on the compressor itself.
Most of the time the answer is something other than a failed compressor. That diagnostic work matters, because the wrong diagnosis means replacing parts that didn’t need replacing.
What You Can Check Before Calling
A few safe things to look at first:
Check the breaker. Confirm it hasn’t tripped. Reset it once and see if it holds. If it trips again immediately, stop there and call a tech.
Look at the condenser coils. If they’re packed with grease and dust, that’s likely part of the problem. Note what you see. A tech will clean them as part of the service call.
Look for ice buildup on the evaporator. If the coil inside the box is completely iced over, the unit may have a defrost system failure or a refrigerant issue. That’s useful information to share when you call.
Don’t test or replace the capacitor yourself. Capacitors hold a charge even after the unit is powered off and can deliver a serious shock.
Don’t add refrigerant. It requires EPA 608 certification, and adding refrigerant to a leaking system without fixing the leak first just delays the real repair and risks compressor damage.
Call a Tech
If the breaker trips again after reset, call a tech. If the unit hums but doesn’t start, call a tech. If you’re not seeing an obvious cause after the checks above, call a tech. Getting a real diagnosis requires gauges, a multimeter, and the experience to put the readings together.
A capacitor or overload replacement is a quick repair. Compressor work is a bigger job, and on older equipment it’s worth a conversation about whether repair or replacement makes more sense.
We handle commercial refrigeration service across the Bay Area, including walk-ins, reach-ins, ice machines, and prep tables. If you’re dealing with this, contact us at bayarearefrigerationservice.com and we’ll get it diagnosed properly.