2003 hyundai elantra

So my 2003 Elantra has been getting horrible gas mileage, running super rich. I had a constant P0420 catalytic emissions code and one misfire reading. I'd clear it and it'd go away for about an hour and come back. I feared I would have to replace my whole exhaust manifold/cat part until I read...

Hey fellas, I just bought a 2003 Hyundai Elantra GLS with ~128,xxx miles on it. I looked up the VIN number and found the car only had 1 owner, got it from an auto auction at $3000. I checked everything out, engine is clean as well as everything else. Took it to a mechanic who inspected the car...