Instructions
Make an appointment with your veterinarian. One of the most common reasons for a cat to suddenly avoid using the litter box is a medical disorder, such as an infection of the urinary tract, kidneys or bladder.
Avoid scolding or punishing your cat. Unless you catch your cat in the act of eliminating outside of the litter box, no amount of correction or redirection is going to help. In fact, your cat will have no clue as to why you have suddenly "lost it" and may even learn to fear you. However, if you do happen to come across your cat in the process of eliminating somewhere she shouldn't, then immediately (and calmly) place the cat in the litter box. This helps to retrain a cat to use the litter box by re-establishing an association between the litter and the urge to go.
Look for stressful factors in your cat's environment. Believe it or not, cats can suffer from stress caused by a change in residence or the introduction of a baby or another pet. Take the time necessary to help your feline friend become acclimated with his new surroundings or new member of the household. Take your cat for frequent visits to the litter box during this period, which should be placed in an area that affords reasonable quiet and privacy.
Use litter designed to please a cat, not people. Some brands of cat litter contain materials that a cat wouldn't normally care to dig into, such as special crystals to alert you to the presence of odor. Assuming that your nose will sufficiently convey this information, opt for a non-scented litter made from natural materials. Making this simple change may be all that's needed to retrain a cat to use the litter box.
Impose temporary confinement, if needed. This may become necessary for a few days or weeks in order to retrain a cat to use the litter box. Choose a safe place that has ample room for your cat and a litter box that can be closed off from the rest of the house. Be sure to provide a bowl of water, but never food of any kind. This is vital since cats will not eliminate where they eat. Check on your cat at regular intervals. When he uses the litter box, grant a reprieve from captivity until it's time to use the litter box again, perhaps after the next feeding. Repeat this process until your cat demonstrates regular use of the litter box once again.
How to Retrain a Cat to Use the Litter Box
As a kitten, your cat received "potty training" from his mother. Hopefully, knowledge of how to use the litter box was retained after being adopted into your household. Still, adult cats sometimes miss the mark and start leaving "surprises" in inappropriate places. The good news is that with patience and persistence, it's possible to retrain a cat to use the litter box again.