Check engine light came on while driving

Check engine light came on while driving. I got a list of problems but I'm still not sure. Couldn't be o2 sensors because it doesn't run up my gas and then when I go up a hill it doesn't affect the car so it's not the catalytic con. But I noticed something weird… When i put my leg to muffler it's not smooth how it used to be? It's like puffs in a way…. Oh and I do have to get a gasket for motor (oil) that's about it. Thank you.
My car has an automatic transmission.

– Check engine light came on while driving