hello scotty,
i have a toyota corolla 2012 1.8L automatic, the chekengine is on. the scanner shows the P0420 code.
How can I know if it is the catalyst or an oxygen sensor?
Do the cheapest things first. If the vehicle is running "rich," that could be tripping the P0420 code. Check your engine air filter, clean your MAF and Throttle Body (see the video), and check for any (even small) EVAP leaks. (Additional air would produce a richer fuel mix, and that could trip the code).
After that's all done, erase the codes, go through the drive cycle and see if the code comes up again. Often it does, but sometimes these cheap actions are enough.
https://www.youtube.com/watch?v=NWCKvU2FmDc
https://www.youtube.com/watch?v=zMok2y05jNE
Many things can cause a specific code, see here:
https://www.autocodes.com/p0420_toyota.html
https://carkiller.com/scottykilmer/qa/trouble-light/
