Boxer urine smell after carpet shampoo failed

A carpet shampoo machine is exactly the wrong tool for pet urine — it spreads the urine salts deeper into the pile and backing, and once that dries, the smell locks in. The foam residue you're seeing is just surfactant that didn't fully rinse, which will keep attracting your dog back to the same spot. The only products that actually eliminate pet odours permanently are enzyme-based cleaners — they digest the uric acid crystals that cause the smell rather than masking them. Before I point you to the right one, can you tell me: is this one specific patch or multiple areas, and roughly how long ago did the original accident happen?

That makes sense — the shampooing has likely pushed the uric acid crystals further down into the carpet backing, which is why it smells worse. Three weeks old plus a shampoo treatment means you're dealing with a well-set stain. Before you apply anything new, you need to get that foam residue out first — just work some plain cold water into the patch and blot it up thoroughly with a dry cloth. Dry it as much as you can. Then I want to ask: is your Boxer neutered or unneutered? Intact males have much more concentrated urine and you may need to soak the area more heavily.

Good — neutered Boxer urine is easier to break down than intact male, so you're in a reasonable position despite the set-in stain. Once that residue is cleared and the area is just damp, the product I'd go straight to is Simple Solution Extreme Pet Stain and Odour Remover. It's a professional-strength enzyme formula specifically rated for set-in stains, and it's widely available on Amazon UK. The key with a stain this old is saturation — you need to apply enough that the enzyme solution reaches as deep as the urine soaked, so don't be sparing with it. Apply generously, cover with a damp cloth to stop it drying too fast, and leave it for at least 10 to 15 minutes before blotting. Don't use any heat to dry it afterwards — a fan or open window is fine, but heat kills the enzymes before they finish working.

Leave the shampoo machine alone for this patch — it'll disrupt the enzymes and you'll be back to square one. Once the Simple Solution has fully dried, just vacuum the area normally. If the smell is significantly reduced but not completely gone after the first treatment, do a second application in the same way — deep-set stains sometimes need two rounds, especially where a previous product has already been worked into the fibres. After that the smell should be gone completely, and your Boxer won't have a scent marker pulling him back to the same spot.

You're set. Cold water rinse first to clear the residue, then a generous soak with Simple Solution Extreme, keep it damp while it works, and no heat drying. Should sort it. Good luck with the big lad.