I've been around cars my entire life and can't tell you how many bad o2's I've replaced and I've NEVER replaced a "bad cat". I'm not saying it is impossible and maybe there is something special about the newer 4runner cats (although I put over 130k miles combined on 3 different 4.0 v6 tacomas and never had a cat issue, but did test drive one with a bad o2 sensor).
I like working on cars but I could NEVER do it as a career because I am way too honest and interested in fixing what is wrong with a vehicle. My dad who was in the car business and was a mechanic by trade before he opened his dealership has been saying for 15 years that people don't "fix" cars anymore, they only replace parts. I'd start with an o2 sensor and go from there and find a nice friend to help you out, you'll need a couple basic hand tools at best. It's always easy to throw MORE parts at it, once you install it though they aren't going to give you your money back if it doesn't fix it
I like working on cars but I could NEVER do it as a career because I am way too honest and interested in fixing what is wrong with a vehicle. My dad who was in the car business and was a mechanic by trade before he opened his dealership has been saying for 15 years that people don't "fix" cars anymore, they only replace parts. I'd start with an o2 sensor and go from there and find a nice friend to help you out, you'll need a couple basic hand tools at best. It's always easy to throw MORE parts at it, once you install it though they aren't going to give you your money back if it doesn't fix it
