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How to train an outdoor cat to use a litter box
Get an open litter box instead of a closed one.
A closed liter box may look nicer and be better at keeping the smells in, but an outdoor cat may not like it. Outdoor cats are used to the open, and a covered box may make them feel closed-in.
Make sure that the box is big enough. The box needs to be large enough for the cat to be able to move around in.