Pet stains are the most difficult stains to remove from carpet. Once pet urine dries it crystallizes and becomes very sticky. It has the consistency of wet sand. I have cleaned so many homes where I found pet stains just from vacuuming. The vacuum would go over an area of the carpet and it would slow down like it was trying to suck up mud. The carpet was just full of urine crystals which hold a lot of moisture. My Tampa FL customers ask me all the time "What is the best pet stain remover?" I tell them it depends on whether it's dried or not. A recent pet stain that is still wet can be removed by rinsing it with a lot of water. I mean a lot of water. I usually recommend pouring about 1 gallon of water on the urine spot and then sucking it up with a wet dry vacuum, or a series of dry towels. Chem-Dry makes and excellent pet stain remover for wet urine spots, called World Famous Pet Odor Remover. It's extremely simple to use and amazingly effective.
Your finding dry pet urine stains there are no consumer products that I know of that are effective for pulling dry urine out of carpet. At Chem-Dry of Tampa FL we use P.U.R.T (Pet Urine Removal Treatment) on dry urine stains. It breaks down the odor and yellowing by super oxidizing the stain. It's an extremely effective pet stain remover. We treat the stain on the carpet and the urine in the pad underneath to make sure it's completely gone. The before and after photos are from a recent pet stain treatment of dry urine spots in a Tampa FL residence. The pictures show how powerful our PURT pet stain remover really is.