I can remove the O2 sensor from the top of my cat. So I was wondering if I could spray some of your carb/ injector cleaner in there to decarbon the catalytic converter. If not do you have a product I could use to try this procedure. I have run a can of B12 thru the fuel system. I have also done the intake valve cleaning procedure with a competitors product thru the brake booster hose. Then had a valve job done. Intake is super clean for a car with 260000 miles on it. Now I am getting a P0420 code. Cat efficiency below theshold. Vehicle doesn't show any signs of the cat being plugged. Excellent power and performance. Just can't pass inspection with the light on. I have also ran two cans of Catakleen thru it without any results. One can of Quaranteed to Pass by CRC. So as you can see just running these cleaners thru the fuel system doesn't seem to work for decarbonizing my cat. Hence the thought of spraying something directly into the honeycomb to break down the Hydrocarbon build up. I have graphed the STFT and the downstream O2 sensor and the voltages from the O2 sensor are jumping all around instead of staying steady. Inspection has run out and I need to fix it before I can drive it. New cat is $265 plus four hours labor to install $500+ total. I don't feel the cat is actually broken, just dirty.