Carpet cleaning can work wonders on restoring dirty old carpets. They will not look very new after the cleaning, but they will look restored and refreshed. However, there are some limitations on what a carpet cleaning service can and cannot do. Here are some of the stains and smells that carpet cleaning cannot remove, and why.
Even when an ink stain is pretreated, professional carpet cleaners will not dissolve the stain or remove it. They can lighten the stain, but the chemicals used to make pen ink will not respond to cleaners in the same way that many other stains will. You can expect some signs of the ink stain to remain.
Bubblegum is not necessarily a stain, but more of something that is stuck in the fibers of the carpet. A carpet cleaner cannot use the cleaning machine to remove the gum, as the gum will not come out via scrubbing. Most cleaning professionals will recommend cutting the gum out of the carpet or freezing it with ice or dry ice to get it to crack and break up in pieces. Then you can remove the gum.
These smells exist in your carpet because the smells have permeated it from beneath the carpet. It means that the carpet pad and/or the bare wood floor under the carpet pad are rotting or rotten. You can clean and treat the carpet down to the carpet pad, but the mold and mildew that exists underneath is not going to go away. A dry chemical cleaning process for carpets can help reduce how much the mold/mildew multiplies and makes your carpet smell, but not every professional carpet washing company provides this option.
The base chemical in pet urine, particularly cat urine, is ammonia. Ammonia stays in the artificial fibers of carpet because the artificial fibers are extremely absorbent and the ammonia molecules bond to the fibers. Cleaning spots of pet urine staining can help remove some of it, but because the smell is so strong, not all of it can be removed. You may find that within a few weeks of cleaning the urine spots the smell lingers still. That also proves problematic if you still have the pet or pets that soiled the carpet in the first place. Pets can sniff out even the faintest odor of their own urine or competing pet's urine, and continue to urinate in those same spots that were just cleaned.