Nobody thinks about the refrigerator until the milk is warm, and by then the coils have usually been dusty for a long time. The coils are the part that gives off the heat the fridge pulls out of the food. When they are coated in lint, pet hair, and kitchen grease, the machine runs longer to do the same job. It is one of the quietest maintenance tasks in a house, and one of the easiest to forget, because nothing shouts about it until something is wrong.
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Decide who owns it
Before anyone touches the fridge, settle who is responsible for it. In a house where Mom handled the kitchen, it probably fell to her without anyone noticing. Now it needs a name. You might take it on, or you might agree with Dad that he will do it, with you checking the calendar. Either way, put a reminder somewhere he will see it, on the fridge door or the wall calendar, so the job stops living only in your head.
Find the coils
Look at the front of the refrigerator at the bottom. Many models have a grille there that pulls off with a few tugs, and the coils sit just behind it. Other models keep the coils on the back, behind a panel, or underneath, behind a kick plate. Your manual will show which kind you have. Check it before you pull anything, because a few models are harder to reach than they look.
Clean them the careful way
Unplug the refrigerator first. Use a vacuum with a brush attachment to lift the loose dust, then a long, thin brush to reach between the fins. The fins bend easily, so brush in one direction and do not press. Do not spray water or a cleaner onto the coils or near the motor, and do not use a sharp tool to scrape them. Plug the fridge back in and let it settle before you check the temperature.
If the fridge has to come away from the wall to reach the back, get a second person, and do not stretch the water line that may run to an ice maker. Move the fridge slowly, and slide it back carefully when you are done.
Know the warning signs
Watch for the fridge running almost all the time, a louder hum than usual, sides that feel warm to the hand, or food that is less cold than it used to be. Those signs can have several causes, and a dusty coil is only one of them. If cleaning the coils does not change how the fridge sounds or feels, ask a repair technician to look at it rather than guessing. Keep the manual, and note which model number you will give them.