If your walk-in cooler is warm and the condenser unit outside is completely silent, the compressor isn’t running. That’s usually one of three things: a failed contactor, a bad start capacitor, or a refrigerant pressure lockout. Here’s how to think through it.
Start With the Obvious
Check the breaker first. A tripped breaker is the most common first call. Reset it once. If it trips again immediately, stop and call a tech, because something downstream is drawing too much current.
Also check the thermostat or controller setpoint. A setpoint that drifted above room temperature will keep the compressor off all day.
If the breaker is fine and the setpoint looks right, the unit should be trying to run. The fact that it isn’t points to the electrical side or a safety lockout.
Most Likely Cause: Bad Contactor
The contactor is a relay, a heavy-duty switch that closes when the thermostat calls for cooling and sends power to the compressor and condenser fan. They wear out. The contact points pit and burn over time, especially in dusty or greasy condenser environments.
A failed contactor typically means neither the compressor nor the condenser fan gets power, so the whole condensing unit goes quiet. If the condenser fan is spinning but the compressor is silent, that points away from a simple contactor failure and toward a compressor-side problem like a seized motor or a tripped internal overload.
A tech checks this with a multimeter: voltage in, no voltage out across the contactor, coil is fine, contacts are bad. It’s a relatively inexpensive part and a straightforward swap, but the work happens inside a live electrical panel. This is not a self-repair job.
Second Most Likely: Failed Start Capacitor
Capacitors give the compressor motor a boost of current on startup. When one fails, the compressor tries to start, draws locked-rotor amperage for a second or two, then trips the internal overload protector. You might hear a brief hum or buzz from the condenser unit. After the overload cools down, the unit tries again with the same result.
If you hear that hum-then-click pattern every few minutes, a capacitor is a strong suspect. They can also fail without any visible symptoms, no bulging, no burn marks. A tech checks them with a capacitor tester; a multimeter resistance check alone isn’t reliable enough.
Capacitors hold a charge even after power is disconnected. Handling them without a proper discharge procedure can cause serious injury. Leave this to a tech.
Third: High or Low Pressure Lockout
Most commercial refrigeration compressors have a high-pressure cutout and a low-pressure cutout. These safety switches prevent the compressor from running when refrigerant pressure is outside safe limits.
High-pressure lockout usually happens when the condenser can’t reject heat: dirty coils, debris blocking the fins, or a failed condenser fan motor. The high-side pressure spikes and the switch opens.
Low-pressure lockout happens when there’s a refrigerant leak. If the system has lost enough charge, suction pressure drops below the cutout threshold and the switch opens to protect the compressor.
Some pressure switches reset automatically once conditions return to normal. Others require a tech to physically reset them after diagnosing the root cause. A high-pressure switch that trips should be treated as manual-reset territory: find out why before clearing it. And if the cause is a refrigerant leak, that’s EPA Section 608 certified work, not a DIY fix.
What You Can Safely Check Yourself
- Breaker. Reset once, watch it.
- Setpoint on the controller.
- Condenser coils. If they’re caked with grease or dust, that’s a likely high-pressure lockout. Don’t run the unit until the coils are cleaned.
- Condenser fan. With power on and a call for cooling, the fan should be spinning. If the whole condensing unit is silent (fan and compressor both dead), that points to a contactor or power supply issue. If the fan is running but the compressor isn’t, the problem is inside the compressor circuit.
- Evaporator fan inside the cooler. This runs independently of the compressor. If it’s not running either, you may have a broader electrical issue.
Don’t open the refrigerant circuit. Don’t reset a pressure switch repeatedly without knowing why it tripped.
How a Tech Diagnoses It
A commercial refrigeration tech shows up with a multimeter, manifold gauges, and a capacitor tester. The sequence is usually: confirm power at the contactor, check contactor operation, pull and test the capacitor, connect gauges to read suction and discharge pressure, and pull service history if the unit has a controller with a fault log.
Most of the time, this is a 30 to 90 minute diagnosis. Contactor and capacitor replacements are same-visit repairs. A refrigerant leak takes longer because the tech needs to find the leak, repair it, pull a vacuum, and recharge to spec.
When to Call Us
Call now if:
- The breaker trips again after one reset.
- You hear repeated hum-then-click cycles from the compressor.
- The entire condensing unit is silent even though the thermostat is calling for cooling.
- You have any reason to think there’s a refrigerant leak (ice on the suction line, oil staining near fittings, a unit that’s been slowly losing effectiveness over weeks).
- You can’t figure out which of these it is within 15 minutes.
A warm walk-in is product loss. The FDA Food Code requires cold storage at or below 41°F for temperature-controlled foods. Once your cooler climbs above that, you’re on a clock.
We do commercial refrigeration service across the Bay Area, walk-ins, reach-ins, prep tables, and ice machines. Call us or reach out at bayarearefrigerationservice.com and we’ll get you on the schedule fast, often same or next day when we can.