Got a P0420 code on my daughter’s vehicle. Also, P1456 code which indicates a gas vapor leak (it turned out to be the gas cap like usual). I purchased a used cat converter from a local junk yard for replacement. I cleaned it by soaking overnight in dishwash soapy water, then rinsed by running water through it while I removed the exhaust system. Let it drain and blew compressed air through it. Something I saw in a Scotty Kilmer video. Hope this helps.