P0420 and P0030 code

Hi, I am new to this forum I have a 2011 Chevy aveo LT automatic transmission recently my check engine light came on and I took it over to a local mechanic to read, two codes show on my car first being the P0030 code for sensor 1 bank 1 in my manifold, other code being P0420 code, I replaced the bank 1 sensor 1 o2 sensor cost be $177.00 and reset the code, about 2 days later the check engine light came back on with the same codes and I noticed it was hesitating when accelerating and going up an incline or hill it seems to have loss of power. I took it back to the mechanic explained to him that it seemed to do fine when the code was reset but issues returned when light came back on. He told me he read it is running lean and there may be a vacuum leak somewhere that needs fixed. I could not for the life of me find a leak. My question is has any had this same issue and what are my options rather then paying over $800 to replace my cat. I thought about gutting it and tricking the o2 sensor but honestly can't find a guide for doing this on my car as I have a cat on the manifold and I believe a second cat before the resonator. Please give me some advice on how to resolve this I live in Ohio and cats are not regulated and emmison testing is not enforced as I live in the country not the city.

How many miles you got on the car? You don't have to pay 800 for an OEM cat, especially if you're in a non-exhaust-regulated state. You could pay 300 or the cheapest I've found, was $90! Do you have any exhaust leaks/holes? When the car is cool, check for noticeable cracks. You can also take off the heat shield on the exhaust manifold to see if there's any cracks there. I actually have one. Getting it replaced tomorrow!