CHECK ENGINE LIGHT ON.
1 Answer
The ECU doesn't check cat temp. It only checks the oxygen level coming out of the cat. It expects to see a fairly steady reading. If the cat isn't working or isn't heating up, the downside oxygen sensor reading will be all over the place. See this post on what the Po420 or P0430 means: https://ricksfreeautorepairadvice.com/p0420-is-it-the-catalytic-converter-or-the-oxygen-sensor/