It's a 2002 Honda Civic Si K20A3 engine. It's thrown a P0420 twice in the last couple of months. I did a valve adjustment a few months ago and I'm not sure if that's a coincidence. The car has 175K with original O2 sensors as well. I was thinking I'd try replacing the O2 sensors first because a cat is ridiculously expensive. Any other items that might throw this code on this car (in case there's something that commonly causes it on this engine)?