One of the toughest pet odors to remove from carpet is cat urine. Carpet with this problem has a distinct smell. Once you know the smell you don't forget it.
I have used several products in an effort to remove cat urine and the accompanying odor. What has worked best for me is vinegar. I brush a bit onto the fibers and allow the vinegar to soak into the pad as well. After the vinegar sets for a while (I don't have a specific time length.) I shampoo the area and extract the excess water.
I can't say that this procedure always eliminates the odor, but in most cases it does, and the vinegar at least lessens the urine smell.