When working properly, the thermostat turns the compressor on and off. Once the thermostat turns the compressor on, it will run until the temperature inside the refrigerator is lower than the setting of the thermostat.
Make sure that the refrigerator has enough air clearance on all sides. Refrigerators with a condenser coil grid on the back of them need room to radiate the heat away from the coils.
Verify that the door light switch is working properly. Push the switch with your finger to verify the light is turning off. If the light stays on, it will actually warm the interior of the refrigerator. Fix or replace the switch if it is faulty.
Modern refrigerators have larger freezer compartments and therefore run for longer periods of time. They also start and stop very frequently to maintain an even temperature. This is especially true if you have just put a lot of warm food in the refrigerator, or if it is very hot and humid. It can take several hours to get back to the cool temperature you have it set for after putting a large amount of food in it.
If the compressor is not shutting off when the temperature inside the refrigerator is cold enough, you may have a bad thermostat. You may also have the temperature set too high in the refrigerator. Try adjusting the thermostat to a lower temperature. If this turns the compressor off, you had it set too high. With the help of a refrigerator thermometer, you can find a more ideal setting that will keep your food cold, but not so cold that the compressor is always running. If raising the temperature setting doesn’t help, it may be a problem with the thermostat and you will need to replace it.
If your thermostat is working properly, there’s a chance that you have low levels of refrigerant in the sealed system. If this is where the symptoms lead you, you need to contact a professional appliance repair person. Only an EPA certified technician can legally work on a sealed system. If you attempt to do work on this system, you will void the warranty.
If this is a new refrigerator, it will run for a long time, almost constantly, before it gets cooled enough to stabilize the interior temperature. Allow up to 24 hours for it to stabilize. The refrigerator should be at least one half to two thirds full. This will help to maintain a stable temperature inside the unit. The cold items inside will help to keep an even temperature inside after the door has been opened and closed.
Some people prefer to keep a lot of stuff in the refrigerator as ballast, even jugs of water. This helps to maintain temperatures inside so that when the door gets opened and closed often, it doesn’t need to run as much to maintain the temperature inside.