Attempt 1: Bissel
I first tried normal Bissel carpet shampoo, thoroughly wetting, sucking up, and re-wetting the carpet many times in the same setting and it did nothing to remove the odor. Next I bought the Hoover Petplus 2x odor remover solution, which also did absolutely nothing to help the smell. They left virtually no odor masking perfume smell once the carpet fully dried, but didn't remove the urine odor either.
Attempt 2: Nature's Miracle Urine Destroyer:
Next, I tried the Nature's Miracle Urine Destroyer per the instructions on the bottle (not for the carpet cleaning machine). It initially helped much more than the other two products. This product definitely leaves behind a lasting perfume scent, if unobjectionable. After a few days the entire area still smells strongly of urine. I repeated and ended up using all of the roughly 1 to 2 quart bottle on the 4 sqft smelly area with no success whatsoever.
Attempt 3: Out! PetCare Stain & Odor Rmoever:
I bought this at Target for about $4.50, applied liberally with the sprayer--though I was tempted just to dump it on the carpet as my hand was getting sore. I was pretty skeptical but thought it would be worth a try. 24 hours after application, there's just a mild scent of detergent and virtually no smell of urine. It does say it contains bacteria and enzyme solution, which I believe might be the key. Neither of the other solutions I tried mention anything about enzymes or bacteria.
Attempt 4: Nature's Miracle Carpet Shampoo
I tried Nature's Miracle carpet shampoo with my carpet cleaning machine. I don't recall what the immediate effect was.
The salesperson at Petsmart gave me some good ideas once the odor problem is solved--to put down the dog training pads to cover the area where the odor was, in order to catch any follow-up urination in the same spot. Also, I'm considering putting my dog in dog diapers at night to make sure there aren't further accidents.
Well, we finally had to euthanize our dog for congestive heart failure about six months after the start of this urine issue. A month or so afterwards, the carpet odors were undetectable. I'm not sure which product did the "trick," or if it was just a matter of time for the smell to dissipate. It also could have been that somehow our dog was periodically urinating in the same spots without us noticing.