Best bet is to clean your deck with a good urine and stool deodorizer such as Natures Miracle or Simple Solution or Odor Mute etc.
Make sure your dog is not having any pain problems in getting from your deck to your yard.
Vision problems can also impact the dog's willingness to use stairs especially if they are open ones. If your dog is healthy then the next step is to retrain her.
Take your dog out on lead right off your deck and into the yard and make sure she potties out in the yard and praise her for doing so. Do this for a few trips all on lead. Then let her off lead when you get to the yard, then when you get to the stairs etc. You want her to have the habit of trotting out the door to the stairs and going down them to the yard.
Then send her out the door on her own. Be ready with a shaker can or a spray bottle of water (whichever works best for your dog) to correct her if she stops to potty on the deck. Rush out after the adversive and hurry her down the steps to the yard and PRAISE her for being in the yard. If she potties when she gets there praise her more!
Most dogs figure out what you want and if you are consistent she will use the yard and not the deck.
Hope this helps!
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