White vinegar works better on cat urine stains than any other product we've found.

The stains in this photo have been accumulating for 2 years on this commercial carpet. We tried 3 brands of grocery store carpet shampoo and 1 type of the commercial stuff in a machine. I was on my knees with a scrub brush, too. Nothing worked.

This is the "after" shot of the same area. I used white venegar from a spray bottle and the stains almost disappeared instantly. I still used a scrub brush to spread the vinegar. This process takes concentrated saturation, so I had to come back a second time to get the spots I missed at first.

The smell takes a couple of days to dissipate, so it's best not to try this just before you expect company. Also, be sure to test it first. White vinegar can cause a bleaching effect on some surfaces.