2018 WRX - P0420 code

Nov 13, 2023
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I have a 2018 WRX that I purchased used with 35k miles. The car now sits with just over 40k and the CEL illuminated on me yesterday. The OBD scanner came back with a P0420 code for bad CAT efficiency. It seems unreasonable that the CAT would be fried after only 40k miles. The car is stock and has no tune that I am aware of programmed into the ECU. I also exclusively use 93 octane gas from my local Shell/Sunoco stations and never had issues before. I did a few pulls in the car to check for power loss and it seems to reach peak boost under power with no problems. The only difference I have really noticed is that the car seems to idle a little different than before. It seems to jump between 700-1000RPM at stop, even when warmed up. I did recently throw in Gumout AIO cleaner with PEA with my last oil change (around 37K, didnt trust that the dealer changed it before selling) for fuel system maintenance. Could this somehow have triggered something in the exhaust system, possibly sending some of that broken down carbon into the cat/O2 sensor downstream and confusing the car? Any suggestions on how to diagnose this at home before I spend too much money at a shop?

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