So yesterday when I had my cat replaced the mechanic showed me my o2 sensor it was only a week old and already had a light coat of black soot on it. My concern is why ? I had no code except p0420 and the cat has resolved that hopefully. The o2 are switching well to. My buddy that works at oreillys got the cat back and said it looked pretty rough for 4 months. I haven't ever had any engine codes besides the 420 and my spark plugs aren't black any thoughts is appreciated .