Rough acceleration, poor gas mileage, code P0420

[Jaygod]

Trying to diagnose what’s wrong with my car.

I have a 2008 Chevy Aveo with 140k miles on it. Recently I started having issues. First I started feeling the car got rough acceleration and struggles to get past 40 mph. After I get past the 40 mark the car accelerates fine. My fuel mileage also got worse. I mostly drive highway and I average 20 mpg. Also, i ran a basic OBD II scan on it and a P0420 code came out, saying there is an issue with the catalytic converter. Can the catalytic converter be the issue? Or can it be something else that I can diagnose easily? I’m planning to take it to a shop next week but for now I want to see if I can find a fix for this. I think it is important to mention that I had a similar problem a few months ago with rough acceleration and the car stalling, found a huge vacuum leak. Changing the air intake hose fixed it. I also found the spark plug holes were filled with oil while changing my plugs, so I replaced the valve cover gaskets (did not change the new spark plugs when I replaced the valve cover gasket tho). Extra detail, the car shakes when idling, but gets normal when driving. I hope y’all can have some ideas 😓

[Ajax]

From what I gather, that code means you are getting ad readings from the O2 sensor. You could try testing the O2 sensor with a multimeter, then checking the downstream O2 as well.

[Jaygod]

That makes sense, I’ll check it out asap and I’ll update. Ty for the feedback 🤗

[Ajax]

That makes sense, I’ll check it out asap and I’ll update. Ty for the feedback 🤗

If both upstream and downstream O2 sensors give the same values, that would mean your cat is shot and probably needs replacing.

Otherwise if the values don't make sense it's probably the sensors themselves or you have a leak.