Unless you have one of the trendy commercial refrigerators -- which can work surprisingly well in residential kitchens and are designed so the fan runs 23 hours out of 24 -- your compressor and fan should be cycling off periodically. If your refrigerator or freezer is on all the time, the cause could be something simple, such as a leaky gasket or pet fur on the condenser coils, or a bit more complicated, such as a fading compressor, thermostat or fan.
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Turn Up the Thermostat
If you are turning on your refrigerator for the first time, it may struggle to achieve below 40 degrees Fahrenheit in the refrigerator compartment for the first 24 hours. It might seem like a paradox, but instead of turning the thermostat down, turn it up above 40 for a day or so, so the condenser doesn't labor and overheat.
Clean the Condenser Coils
If you have an older refrigerator, start your troubleshooting by unplugging it and pulling it away from the wall. Remove any screws in the grid or grill at the bottom rear of the unit. Look for a black set of concentric rings that resemble a car radiator, likely covered with fuzz, pet hair and general grunge. Vacuum the outside of the coils carefully, with a crevice tool, avoiding any nearby wires. Clean the interior of the coils with a specialty coil-cleaning brush.
Mark your calendar to repeat the coil cleaning every six months.
Change the Door Gasket
Your coil-cleaning project may help the refrigerator operate correctly, since this is one of the problems that can prevent the freezer from chilling to below 10 degrees Fahrenheit. A too-warm freezer will lead to the noise of the condenser laboring nonstop in a vain attempt to drop to zero.
A bad door gasket leaking cold air, which you can test by closing the door on a dollar bill and checking for tension as you withdraw the bill. Repeat the test around the entire gasket. To replace your door gasket, loosen the retainer with a hex head driver, remove the old gasket and work the new gasket into place.
Check the Compressor
A worn-out compressor, most likely in older refrigerators, or a refrigerant leak. These require professional service. Suspect this as the problem if you have tight gaskets but a freezer thermometer registers above 10 degrees Fahrenheit after you clean the condenser coils. If you remove the evaporator cover and half the coils are frost-covered and half are bare, that's another clue that the compressor may be dead or dying.
Get Out Your Continuity Tester or Multimeter
Unplug the refrigerator to check for problems in various electrical components that can go bad over time; you may have to go through each in turn until your multimeter reveals which is the culprit.
Defrost Heater or Defrost Thermostat: Unclip or unscrew the back panel of the freezer, as well as any intervening shelves or icemaker parts, to remove the defrost heater, which cannot be tested in place. Set your multimeter on its lowest ohms setting, and touch one probe to each terminal. Look for a reading between 0 and 50 ohms to indicate continuity, and lack of needle movement or digital display change to indicate a lack of continuity and a need for replacement.
To test the defrost thermostat, similarly remove the back panel, unclip the old thermostat from the refrigerant tubing and cut its wires to free it. Place it in a cup of ice water for two minutes. Zero ohms of resistance indicates continuity. If the needle or digital display doesn't react, the thermostat is bad and needs replacement.
Evaporator and Condenser Fan Motors: The back panel of the freezer is also where you'll test or replace the evaporator fan motor. Remove the screws to the evaporator cover and wiggle the fan motor off its spring clip. Check the two non-ground wires, they may be red and grey, for continuity, looking for a positive ohms reading.
To test or replace the condenser fan motor, pull the refrigerator away from the wall and pull off the protective grid. Disconnect the fan motor's wire harness and unscrew the motor mounts. Rotate the motor and its mount out of the refrigerator cabinet. Place the probes on the motor terminals and look for a reading of zero ohms to indicate a good motor.
Refrigerator Compartment Thermostat: Open the refrigerator door and disassemble the control panel at the top to access the regular thermostat for testing. Remove the thermostat and its knob. Test it for continuity with the thermostat turned up high.