Hi all,

I have a 2015 Mitsubisihi Outlander Sport (210,000 miles) that has had the engine light on for 2 years now. The code is P0421. So it's the cat. The engine was rough idling and had hesitating acceleration, but that isn't noticeable any more after cleaning the throttle body (which was dirty) recently. Now it only rough idles when the AC is on. When the AC is on, the engine cycles between around .6 and 1 thousand rpm in a predictable pattern that makes it feel as though it is struggling. Fuel efficiency is in the 20s mpg. The issue of the cat has made me concerned about what caused the cat failure in the first place. The engine has been consuming oil pretty fast and when I checked the spark plugs (which may be as old as the vehicle) I saw that they had a sandy substance on them both in color and texture. The threads of the plugs were wet with oil. This led me to believe the valve seals may be leaking. There is no noticeable smoke coming from the exhaust at any time. Obviously, I am concerned about putting in the time and money to do these repairs so I wanted some more opinions on what the issue could be since I am new to car repair.
Spark plugs