Thus brings me to my fear lol.If your car is stock I would change the spark plugs every 25,000 miles in spite of what the manual says.
At 50,000 miles you very well could need carbon cleaning. I had the chemical carbon cleaning done once at 50,000 miles and saw before and after pictures and then had manual carbon cleaning done at the next 50,000 miles and I'll just do manual carbon cleaning every 50,000 miles from now on. The chemical cleaning
didn't work that well.
Did you do the carbon cleaning or shop? I Have a CPO can I use the warranty for it?