P0420 on 2025 CHEVROLET Spark

“There may be some other issue afoot. My '09 Forester popped a P0420, and the root cause was not (just) that the catalyst is old and worn, but that the valve cover gaskets were leaking oil into the plug tubes and causing weak spark in cylinder 2 and 4, clogging the cat with soot.”

“Just like OP's, the issues showed up mostly after longer drives. I fixed that, cleaned up the original cat, put it back in and the P0420 has been off for months.”

“Get an obd2 reader they are cheap asf. If it's the p0420 or p0430 code, those are the same ones I get during seasonal change because the catalytic converter efficiency is so sensitive and dependent on temperature.”

“I don't worry about it and I just erase them and they don't come back for 6 months or more. If you're concerned it may be something else (could be inefficient combustion or dirty o2 sensors or failing spark plugs, there's a bunch of actual issues that can cause those codes) then have a mechanic diagnose the issue.”

“Quote: Originally Posted by tony_2018 trying to hold off on spending 1200 for a cat...gotta try something at least. get a spark plug defouler, there is a trick you can look up for fixing a p0420 code.”