My usual mechanic cleaned my fuel injectors with the cleaner and soon after my gas mileage dropped from 25 mpg to 18 mpg. I took it back to that mechanic to see what was up and he couldn't find anything. It didn't get any better so I went to a different mechanic who said my o2 sensor was screwed up. He replaced that and said my cat might be clogged from when the cleaner removed all the junk from my fuel injectors. He thinks all that crud went down and are now clogging my cat. He said there is a possibility it could clean itself, but otherwise I need a new one. I'm supposed to run 3 tanks of gas through my car to see if the gas mileage gets any better. If I need a new cat it will cost about $350 in parts and another $100 in labor. ]

My question is, is this common, or is he just trying to make money off of me? I have a friend who is looking online for cheaper cats, but I'd still need to have someone put it on if I got it. Is it something easily done (not by me, but by my friend who has intermediate mechanic skills) or do I need to stick to a mechanic?