Coprophagia is feces-eating. It is natural for a mother dog when nursing puppies to encourage defecation and keep the nest clean, but it's not appropriate otherwise. Rule out medical conditions with your veterinarian, and use methods of reconditioning your dog such as feces removal and positive or negative reinforcement to break the waste-eating habit.
Medical issues can cause coprophagia. Your vet will do tests to rule out any type of disease that could cause your dog to eat stool, including Cushing's syndrome, diabetes, intestinal parasites, hyperthyroidism and malabsorption syndrome. Some dogs on corticosteroids may eat stool.
Every time your dog goes outside to relieve himself, go with him, even if you have a fenced yard. Take a baggie with you and immediately pick up feces to dispose of it. If you have multiple pets, pick up behind each one when they go potty outside.
Teach your dog the Leave It command. When the dog approaches an inappropriate item, command, "Leave it," and distract the dog with a toy that gets his attention. Praise and treat the dog, and give him the toy. Some dogs learn quickly and will respond to a sharp verbal command quickly. Others are more stubborn. Leash a stubborn dog and bring him near feces. Give the Leave It command and restrain your dog from reaching the feces. Give him lots of love and a treat when he relents. You'll have to practice in various places on-lead until the dog responds to the Leave It command reliably.
Try using a loud sound from a distance to startle your dog when he sniffs stool and before he eats it. An air horn should do the job, but do not use it too near to him -- it can affect his hearing. And it will likely disturb your neighbors, if they're close.
Changing the Taste
Taste deterrents designed to keep dogs from mouthing items are generally very bitter, so dogs avoid them after a taste. Spray a deterrent on every stool to keep your pooch from eating them. Meanwhile, add to your dog's food such items as canned pumpkin, meat tenderizer, crushed pineapple, sulfur or sauerkraut. These foods change the taste and odor of a dog's stool. Many dogs will not eat stool with these items in them. Pet stores carry products to put on your dog's food especially for this purpose.
Litter Box Tips
Move your cat litter box into a smaller room, such as a laundry room or bathroom with a door. Place a baby gate in the doorway to keep your dog from entering the room. Your cat can jump over the gate to do his business. A hooded litter box will keep some dogs out of the litter, but it's not a foolproof deterrent.