Check the door gasket for cracks, holes, or damage. If it is damaged, replace it immediately with a new gasket. A damaged door gasket can allow cold air to escape from your refrigerator and cause it to work harder than normal. This will increase your energy costs and shorten the life of your appliance by forcing it to work harder than it was designed. If you have an old refrigerator that isn't cooling properly (and doesn't seem like anything else is wrong), check its gaskets first!